You generally wear the same outfit the whole race, called a tri suit. It's pretty tight, to avoid drag as you swim and bike, but quick drying so that you aren't water logged after the swim. It has a padded chamois, like bike shorts, but much less padding, so it dries faster and so you don't feel like you are running with a diaper on. You are labeled, by marker, or temporary tattoo, with your number on your biceps and quads, and perhaps your age group on a calf.
Most triathlons have open water swims. The start is often staggered, so there aren't too many people swimming in a clump at once, though it still gets congested. You are required to wear a bright swim cap with your number. You want googles as well. If the water temp is below a certain temperature, you can wear a wet suit without being penalized (the buoyancy makes you faster), but no wet suits are allowed above 78ºC. When your wave starts, you run into the water, swim around the buoys as instructed, and come out where indicated. The swim distance in the races I will be doing this summer is a half mile.
You then run into the transition area. The next segment is the bike. You put on your shoes (and socks if desired), sunglasses (if needed), and helmet. You then run your bike out to the mount line until you can get on and start riding. The Running Fit races have a bike distance of 12 miles.
Once you reach the dismount line (usually the same as the mount line) you have to jump of your bike and run it back to your place in the transition area. You place your bike on the rack, take off your helmet, change your shoes, and grab your race tag. Then it's just the running segment, in this case a 5k (3.1 Miles).
You are timed on all segments of the race; swim, transition 1 (T1), bike, transition 2 (T2), and run. The total time determines the winners, so you need to be quick at all aspects to succeed. We'll be practicing more of the transitions and starts in the triathlon class I started this past week.