Basically, there are two types of math if you will, applied and theoretical mathematics. Computer science uses more of discrete mathematics and numerical approximations while more traditional mathematics is more about proofs and theorems. It is not that one is better than the other, it is just a...
Technically, a function must pass the vertical line test or that you cannot have two y values for one x value. A function usually involves two or more variables such as y=x^2+5.