As an introduction to the sport of triathlon, let's look at the distances of the most common types of triathlon, and how these different distances might play to different kinds of strengths and weaknesses.
In triathlon, the usual order of events is: Swim, then Bike, then Run. So, we will list the distances at the top of each section in that order. Also, since triathlon is a truly global sport now (with Aussies, Kiwis and Europeans dominating the sport), I will list distances in kilometres, as well as miles.
Super-sprint - 400m (~1/4mi), 10k (6.2mi), 3k (~2mi)
This is one of those distances that doesn't really have any kind of "official" measurements, so what I am writing down here is just an example of a fairly common distance you might find for a super-sprint. But, you can also expect to see a lot of variation. 400m in a swimming pool tends to be pretty easy to organise for most race organisers, but the 10k bike and 3k run can vary a lot, depending on where the race is held.
Super sprint can be a great entry-level triathlon distance for your very first race, if you just want to "dip your toes in the water" of triathlon. You could expect to get properly prepared for a super-sprint triathlon with only a couple/few months of consistent training. For these sorts of distances, especially if it is a pool-based race, you probably don't need to buy any new equipment for this. I think you will also find that most people entering this sort of a race are either first-timers (who will be as nervous as you are) or experienced triathletes just doing it for fun, so the atmosphere would tend to be fun and very supportive.
The down-side of a super sprint is that it does favor fast people with high anaerobic capacities, especially younger athletes. These sorts of distances tend to be more common amongst teenage and junior triathletes (although they probably will be given a separate start time from the older athletes, so there is probably no need to worry about being overtaken or lapped by a bunch of kids).
Sprint - 750m (~.5mi), 20k (12.5mi), 5k (3.1mi)
This is actually a standard race format, for which there are national and international age group championships, that would aim to be as close to these standard distances as possible. Of course, this is also a very attractive distance for first-timers, and you may find a lot of local clubs putting on sprint distance races with some variations from these standard measurements. For example, you might also find a lot of local clubs putting on a sprint-distance race in a pool (it can be much more challenging and expensive to host a race with an open-water section than it is to have a pool-based section), and so they may shorten the swim distance considerably, but keep the bike and the run sections as close to standard as they can.
Similar to a super-sprint, this can be a very attractive distance for people new to the sport and you might expect to get ready for a sprint-distance triathlon with only a few months of consistent training. You probably wouldn't need to buy any new equipment for a race like this, unless it were an open-water swim and you don't have a wetsuit.
And, similar to a super-sprint, the main down-side of sprint races, from a race performance point of view, is that they also tend to favor very fast athletes with a high anaerobic capacity, like teenagers and juniors. At this distance, as well, transitions are very, very important. Generally speaking, the shorter the race, the more important transitions are for your overall race time. For example, if you ever did a half-iron- or iron-distance race, you would find that most people have no worries about taking 5 minutes in transition to dry off, change their clothes and equipment for the next section and make sure everything fits comfortably and snugly. Whereas, in these shorter distance races, taking 5 minutes in transition would be considered glacially slow and devastating to your potential race performance.
*Let's also take a moment and put a few things in perspective. The names of different distances in triathlons tell you a lot about the sport and it is worth considering this for a moment. For example, we call it a "Sprint Triathlon," but these distances are anything but sprints. Talk to a swimmer and tell him or her that you are doing an event called a "sprint" with a 750m swim called a sprint, and they may laugh and tell you that you don't sprint for 750m of swimming! Or, tell a runner that you are just doing a "little sprint race" with a 5k run at the end of it and perhaps their eyes will widen and look at you to make sure you know what you are talking about. What all this means is that, by definition, triathlon is an endurance sport, and our shortest of distances in our organised races are actually considered by many to be middle-distance races that require years of training and preparation. So, from one point of view, these distances discussed so far are attractive, shorter distances to encourage younger athletes, people new to the sport, or just people who prefer these shorter distances to the longer ones we will discuss below. But, from another point of view, these distances involve sports training that many people take very, very seriously and work very hard for years to achieve some kind of noteworthy performance.
Elites will typically finish a sprint triathlon in about an hour, and the faster age-groupers will typically finish in about 1hr 15min or so. So, any sort of activity that takes an hour or more of effort and attention is most certainly NOT a sprint.
Olympic - 1.5k (~.9mi), 40k (~25mi), 10k (6.2mi)
This is called Olympic distance because it is the distance of triathlon that is raced at the Olympics. Many people also refer to this as "Standard distance," to avoid improperly using the name of the Olympics in a sporting event. This is the sort of distance where you might start to be taking the sport of triathlon seriously, as it would typically be recommended that you give yourself a good, long amount of time to train for a race (typically, 6-9 months would be a good minimum training time before the race). And, you probably want to make sure you have equipment for your race that you will be comfortable and well-practiced with - not something that you borrowed, rented or bought at the last minute. For example, riding a bike that doesn't fit you for 40km or 25 miles, or without padded cycling shorts (or a padded tri-suit) can be a very, very uncomfortable experience, as can swimming 1.5km or about a mile in an ill-fitting wetsuit.
So, typically, this is not a good race distance for first-timers, unless you already have some experience of training in triathlon disciplines (perhaps not together in one event, but perhaps you have swum competitively, cycled with a club for 20+ miles, and run a few 5-10k races or a half-marathon before). This is the sort of race most people would want to build up to over time.
Of course, I have seen people (and cursed at them) who have shown up for an Olympic distance race who laughingly tell anyone who will listen that they did absolutely no training for this race and went out to a party last night, so they are just going to go out and "give this triathlon thing a try today," as they line up at the starting line - in the same way that you hear people showing up for marathons sometimes do. These are usually the same people who are pale, dehydrated and struggling to find the nearest aid station somewhere in the middle of the race, and not actually having a lot of fun on the day. It's not that triathlons at these distances should be scary, it's just that you should respect your body and what you might be putting it through when you tackle certain challenges.
Elites will finish an Olympic distance in under 2 hours and the faster age-groupers will finish in under 2hr 30min. So, this is the sort of effort you might expect from something between half-marathon to marathon-distance training and racing.
Middle distance (or half-iron) - 1.9km (~1.2mi), 90k (~56mi), 21k (13.1mi)
I will only refer to this as "Half-Iron" and many people refer to this as "Middle Distance." I will only use the word "ironman" (R) once here, because that word is registered by the World Triathlon Corporation, and that is a fundamental concept with which I strongly disagree. I do not believe that a corporate body should be allowed to copyright or trademark a word that has for years been used as a generic description of a sports activity, any more than anyone should be allowed to copyright or trademark the word "marathon." Anyway, that's my opinion on that subject.
Half-Iron races tend to require much more serious training and equipment purchases. I would typically recommend that people build up to this sort of distance over the course of at least 2-3 years, if not longer. I have also noticed that this is one of the fastest growing areas of triathlon, with the World Triathlon Corporation adding more of these (what they call 70.3) middle distance races to their calendar than any other category, each year. As such, I have also seen more and more people who are new to the sport trying to tackle this distance as their very first race, or at least in their very first season of triathlon racing. I don't advise it as a general approach, but it is not impossible, if you intend to train very consistently and spend a lot of money on the sport. Let's face it, the races themselves are more expensive ($150-400 per race entry, perhaps even as much as $600 if you miss the general registration rounds and enter with a charity placement). And, you are definitely don't want to ride 90km or run a half-marathon with equipment that isn't comfortable, properly fitted and well-practiced.
Half-Iron distance races do have one interesting sort of advantage for some triathletes, which is that, of all the distances, they tend to favor weaker swimmers. If you look at the progression from Olympic to Half-Iron, you will notice that the jump from 1.5k to 1.9k for the swim is relatively small, while the jump on the bike from 40k to 90k is a much larger relative increase, and the jump on the run from 10k to 21k is also a more significant increase. So, if you are like the vast majority of triathletes and swimming is not your strongest discipline, you may find some day that Half-Iron races might suit you more than other distance races.
Elites will finish a Half-Iron race in 4-4.5 hours, and faster age groupers will finish in 5-6 hours. So, take this one seriously, and train accordingly.
**It is also worth noting that the nutrition and hydration elements of racing at these distances really becomes critically important at these sorts of distances. As a coach, I would argue that hydration and nutrition are critically important at all distances, but I notice a lot of athletes seem to get away with not paying a lot of attention to those things at Sprint and Olympic distances. But, no one can avoid paying close attention to hydration and nutrition, in their training and in their racing, at this distance.
Long distance (or Iron-distance) - 3.8k (2.4mi), 180k (112mi), 42k (26.2mi)
Yes, that's right, you will end your Iron-distance race by running a marathon in its entirety. This is the serious stuff, here. Iron-distance racing requires a whole separate blog, in itself, to do the subject justice. But, I will just make a few comments here, and you can read more later if you are interested.
Typically, I recommend that people build up to doing Iron-distance races over the course of at least 2-4 years, depending on what kind of sports background they come from.
With the growth in popularity of the sport of triathlon around the world, and the growth of the World Triathlon Coroporation in promoting their own, increasingly popular races, this category of Long distance or Iron-distance races has grown enormously, too. I have also seen a growing number of people who want to tackle one of these races in their first season, and I strongly advise against it. It is not impossible, and some athletes have accomplished their goal of doing an Iron-distance race as their very first ever triathlon, but it is not advisable for most people to consider this, even a little tiny bit.
The elites are going to finish an Iron-distance race in about 8-9 hours and the faster age groupers will finish in around 10-12 hours. It is an all-day event for most people. It is also an all-consuming effort to train and prepare for an Iron-distance race. It is certainly something not to be taken lightly, physically, psychologically, emotionally, financially and personally.