What Makes a Golf Ball different from others?

I work at a sporting goods store and now since are golf pro is gone they put me in golf for a bit. The problem is I know the bare minimum about golf but really want to learn about it. I look at the whole wall of golf balls and I just wonder what makes one better than the other? Is it player preference?

I played college golf and worked at the King & Bear of World Golf Village and took the P.A.T. once and passed it very easily. I keep thinking I should become a PGA Professional because that is all I like besides sex, LOL. The thing that keeps me from pursuing it is that the pay is awful. I know these are a bunch of questions, but any suggestions from a golf pro would be helpful. Thanks!