Question

nPr and nCr; what are they? How are they used in probability?

An explanation of why they work would be good too,

A link to somewhere for me to read for myself would be fine.

Answer 1

They are counting functions.

Answer 2

For nCr, you have a set of items with size n. You want to pick out r items. The order in which you pick the items out does NOT matter. (You don't want to double count a different ordering)
For nPr, you are doing the same thing but the order does matter.