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.

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.

anonymous · Answer

They are counting functions.

anonymous · Answer

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.