To answer this question, let's go straight to the horses mouth (Tyra's the horse of course, cuz there's no other horse of course, than the fabulous Ms. Banks):
"A top model is someone who lives and breathe fashion. She's someone who can do high fashion , and then turn around and whore herself on the cover of Sports Illustrated. A top model has no shame, she'll do back up dancing, and serve as escorts for my friends in the fashion biz. A top model is edgy, and commercial. But not too edgy. And not too commercial. A top model must be sexy. A top model must not be sexier than me, however. I hope this clears things up."