I understand that for a given combination of n objects, there will be n! permutations, and also understand how there is nPr ways to choose r objects from a combination of n, given by:

$\displaystyle ^nP_r=\frac{n!}{(n-r)!}$

I don't however understand what nCr represents, and how it relates to its respective formula, which looks like a simple modification of the formula for nPr. Can someone try to explain? Thanks.