Assuming you have some sort of pool of work to choose from— it really depends on the art school. Some art schools have very specific requirements for their portfolio, in which case it is up to you to research those requirements. Others merely give a number limit of works and general guidelines. Some do a combination of sorts. You have to be familiar with each school’s requirements.
I’m only knowledgeable about drawing/painting portfolios. So I’ll assume that’s what you’re going for. If you’re a film/photo/fashion design/architecture/etc., I hope you don’t mind me taking this opportunity to talk about drawing/painting portfolios for a bit.
A lot of art colleges like to see drawings/paintings from observation, just as a demonstration that you can draw. (You’ll be doing quite a lot of drawing from observation in art school anyway. Might as well get practice in :) )
Some colleges like the more traditional stuff, others want to see more conceptual. I went to portfolio reviews with mostly traditional. I heard different advice from each school, with positive feedback on the traditional part of my portfolio. Of course, the schools more focused on the conceptual side said they would like to see more of that.
It really depends on the school. I highly recommend doing portfolio reviews or NPD (national portfolio day) so you get an idea of not only where your portfolio stands, but what each school is looking for.
As a general rule for any type of artist, show your technical competence and also include works that show who you are, and how you go about making art, works that show your voice. Always try to include your best (i.e. don’t include something in a different medium just for the sake of including more media, if it’s not as strong.)
Regardless of which type of artist you are, admissions counselors/portfolio reviewers can give you the best advice. Even if you don’t have a portfolio put together, it’s still perfectly okay to ask what kinds of works you should include. It shows you’re actually thinking about what you include, which is nice.
Uhh I hope this was helpful and not just redundant of things you already knew. If you have more questions feel free to ask on my personal tumblr, or un-anonymously here so we can have a private conversation. Of course, I’m just a high school student, so I can only offer what I’ve experienced from my own portfolio reviews.
Few more notes: colleges don’t like to see anime, copied works, etc. Avoid those. Show what YOU can do, what YOU can make, not what you can imitate.
A lot of admissions counselors can tell if you drew/painted something from observation or from a photograph. They can tell. Do it from observation when you can.
And if an art school doesn’t ask for a portfolio as an admissions requirement, that’s very worrying.