My dad has been cheating on my mom and is unaware that I know.  Do I confront him?  Do I tell my mom?  What do I do?

Stay out of it. Period. End of story. No further discussion necessary.

This does not mean you don’t love your parents. This does not mean you don’t care about both of them. This does not mean you endorse one parent’s behavior while decrying the other’s.

It means, simply, this is your parents’ problem and it’s up to them to solve.

What you want to master is the art of loving non-attachment to their relationship result. Don’t allow your parents to make you choose sides (unless one or both parents are engaging in abuse). Focus on what you require from both of them as proof of their love for you. That does not include ending the affair, staying married, or getting divorced. Those decisions have nothing to do with you. I’m talking about them being present for your concerns and wishes and showing that concern for you in a tangible way. Any time either one of them attempts to enlist you in the “Who’s right?” and “Who’s wrong?” game, you’ll respond with “That has nothing to do with me. Be the loving parents to me that I expect.”

This will make it absolutely clear to them that any of their relationship problems will have to be dealt with themselves, by counselors, and/or through attorneys. None of those people are you and you should never serve in those capacities, no matter how badly they’re hurting.

This sucks for you. All the best in maintaining your boundaries. Should you need a safe individual to express your concerns and problems with, find a professional, credentialed counselor of your own (not a family member or friend that will take sides).