| The definition pf permutataions and combinations should be in your text book. Briefly, P(a,b) is the permutation of 'a' things taken 'b' at a time. Typically used to calcualte how many different ways one can order 'b' items selected from a group of 'a' items when order is important. It is calculated: P(a,b) = a!/(a-b)!
For combinations, it's the permutation divided by b!: C(a,b) = a!/[(a-b)!*b!]. Typically used to determine how many ways one can select 'b' items from a group of 'a' when order is not important. | 
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