The Fundamental Counting Principle

If one event can occur n ways, and another can occur m ways, they can jointly occur in m*n ways.

Permutations

Permutations -- Order Matters!

Permutation: (ABC and BCA are not equal)

Permutations taken in groups: the number of permutations of n objects taken r at a time.
nPr = n!/(n-r)!

Combinations

Combinations -- Order has no place here!

Combinations: (ABC = BCA ­ there is no difference!)

Combination: is an unordered arrangement of r objects taken from a set of n possibilities.

nCr = n!/(n-r)!r!