I figure as good a place to start as any is to give you an idea of typical race distances for triathlons. There are 4 varieties of triathlon you see more than any other: Sprint, Olympic, Half-Ironman, & Ironman.
1) Sprint. Typical distance: 300m(~330 yd) swim, 20k(~12.4 mile) bike, 5k(~3.1 mile) run. You'll see some variations on this level depending on the course the RD(race director) puts together, so you can see anything from 300-500m swim & 12-15 mile bike. Usually the run stays right at 5k, though I've run a couple that were a little shorter. Most sprint triathlons are a pool swim, but it can be open water too. Most likely you'll see it open water if they're running a sprint at the same time as a longer distance tri.
2) Olympic. Typical distance: 1500m(~1640 yd) swim, 40k(~25 mile) bike, 10k(~6.2 mile) run. There's some variation here too, but it's less likely than in the sprints. Why? Because the sprints are more for the "casual" - and I use that term loosely - triathlete so they're not quite so OCD about distances. In an oly you might see the swim vary by a couple hundred meters, but the bike & run are usually the same as I listed above.
3) HIM. Typical distance: 1.2 mile swim, 56 mile bike, 13.1 mile run. The HIM distances never vary because the triathletes at this level are more likely to be serious competitors. Perhaps I should qualify that: most probably aren't trying to *win* the race, but they're all serious about the competition and about their times. The official HIMs are used as qualifying races for the full IMs as well, so the distances have been standardized for that reason. Training for triathlons also starts to get very specific at this level, with people following detailed workout regimens. OK, other people follow detailed workout regimens. I'm freelancing, which will probably be the end of me come October.
4) IM. Typical distance: 2.4 mile swim, 112 mile bike, 26.2 mile run. Yea, that's a marathon at the end. This is the race for crazy people, which includes my girlfriend(but she's crazy in all the right ways, heh). When you see the Hawaii IM, that's the championship IM. Most triathletes never make it that far. There are official(and unofficial) IMs all over the world, and most of the official ones sell out the day they open for registration. The ones that don't, they sell out in a week.
There are other types of triathlons that show up from time to time, like the reverse tri(or runbitri, they run then bike then swim) or the indoor tri(usually only happens up north where it's cold) and the extreme ones like the double IM or triple IM(seriously, they're out there). Those tris are relatively rare though, 90% of the triathlons you see fall into one of the 4 categories I detailed.
Those are the basic distances. From here on out if you see me reference a sprint, oly, etc., you'll know what I'm talking about.