Very basic.
Vision 20/20 (with or without glasses), hearing, EKG (over age 35), some basic poking and prodding. Also a blood sugar check (diabetes).
They will have you fill out a form about your history also...this is a little tricky. Some past problems can raise a lot of flags, but you DO NOT want to lie on the form. If you have any "issues", make sue they are either old history or stable before you go in. And bring plenty of documentation.
Note: some airlines give EXTREMELY thorough medical exams.