So what do they ask you ?

Yes, music schools require an audition - or at least the four-year accredited ones all do (some unaccredited ones may be more interested in your cash than your chops). Graduate programs will also have an audition for performance majors, but for other types of majors they'll sometimes substitute something else - composition applicants might submit a portfolio of works, theory applicants a term paper they wrote, etc.

For a typical entrance requirement you'll have an audition, and they'll give you a list of audition pieces in advance. Most schools have these lists somewhere on their websites. You pick one (or two or three) from that list. Some schools have you do one from a list and one of your choice; others have you pick one from each of different lists, often arranged by era - say one baroque, one classical/romantic, and one modern. Some schools will let you do that part by submitting a recording instead of attending a live audition.

At a live audition they may also require you to play certain scales from memory, or do a bit of sight reading.

There are usually two other tests: keyboard skills and music theory. These are more placement exams... the audition gets you in, the tests determine whether you're placed in the standard, remedial, or accelerated tracks for the 'musicianship' courses. These tests can vary a great deal from one place to another; one might include questions on music history and important composers, another might have a section where you have to identify intervals or chords by ear.

The questions themselves are going to vary from pretty easy (say, writing out a major scale in a specified key) to pretty hard (analyzing the chord progression in a four part choral piece) because they're trying to figure out gaps in what you know.

Oh, and most schools are also going to want information on who you've studied with, any awards you've won, etc, and maybe some letters of recommendation from past teachers. I'd guess this isn't much of a factor if you ace the audition, but if you're close to the cut-off for enrollment they might use it as a tie-breaker to decide who gets in.