There are many options on the market for tri-specific wear during your race. That doesn’t mean that you need to buy it to race. It can have its advantages, depending on the race course and length, but if you want to try a tri but don’t want to buy new gear, that shouldn’t stop you!
Triathlon clothing is designed to be worn for the entire race – swim, bike, and run. If you wear a wetsuit in the swim, that would go over the tri wear so that you can pull off the wetsuit and you are ready to go. (With the exception of some long distance races, you are not allowed to fully change between swim, bike, and run even if you wanted to because nudity is prohibited).
Tri shorts have some kind of padding in the crotch. It isn’t anywhere near the padding in most bike shorts, but it is something. You wear them without underwear, and they will be quick dry, though there is some variety of material on the market. One difference among options on the market besides general sizing is how they design the bottom – the part that fits on your thigh. They have to be designed, or should be, to stay down on your leg rather than to ride up; so there is usually some kind of sticky plastic and extra elastic to make this happen. In the past I have gotten chaffed by the material in this area. Body glide or something similar can help. Just something to be aware of; but I think it has to be trial and error to some extent to find what you like best. Some tri shorts will also have little pockets in the back to store stuff (gels? etc) during the race.
Tri tops, both male and female, are basically just a super thin, quick-dry, very tight tank top. Some have sleeves, but most do not. You may be able to find one with pockets. Some for females will have a built in bra. That doesn’t work for me at all so I have no experience in that area. I just wear a regular old sports bra and do not buy one with a built in bra which will just get in the way. One problem with tri tops that I have – in part because of my big belly, though I have heard that others have the problem as well – is that it rides up constantly. Some people overcome this by installing a couple of buttons to attach the top to the bottoms.
Or…you can just go for a full tri suit. This is a one-piece suit. Same type of materials and same basics as mentioned above. This has been my favorite option for awhile now. It is just easy. The one very obvious problem though is going to the bathroom. Some people, at least in an Ironman, will just go to the bathroom on themselves while racing. I haven’t figured out how to do this yet and I’m not so sure I want to. I have been able to peel myself out of my suit and get it back up even when insanely sweaty and sticky. My current, and favorite suit so far, has a zip down the front and sleeves. [Update: It seems that the day after posting this the link no longer works. It appears that Coeur has sold out of my fav. Fingers crossed they bring it back though I fear that they will not. Sad day.) Both features that I love, and it has my favorite type of fabric around the base of the shorts on my thighs.