Mostly, capital gains taxes. Long term or short term depending on how long you've held the asset. You may be assessed these by both the US federal government and your state of residence. (The situation is more complicated internationally... as always.) In the United States, you almost always want things structure such that income is classified as capital gains rather than ordinary income, because your effective tax rate will almost certainly be lower that way. (This is, again, almost certainly what will happen as a result of your exit. That's not totally guaranteed, though -- just like we geeks are pretty good about getting integers to behave like strings when we need them to, a good lawyer or accountant can often propose a way to structure a transaction such that it has desirable properties.)
Talk to an accountant before you found a company (goes for you and for everybody else) and talk to them again before and after the sale; there are some subtleties.
Ask about "qualified small business stock" if you're pretty sure this is not your last rodeo and you've been doing your startup for 5+ years.
An example of a good reason to talk to accountants prior to doing things: I invested a very small amount of money in a tech startup. My accountant suggested that I consider investing through a self-directed Roth IRA, which would (if the company IPOed) let me avoid paying any capital gains taxes on it or any investments made subsequently with that money (if I were willing to wait until retirement to touch the funds). In the event that retirement wasn't an option, there's a plan B: the magic words are "substantially equal periodic payments" and your accountant can explain the calculation to you.