Permutation implies arrangement where order of things is important and includes word formation, number formation, circular permutation etc.
Combination means selection where order is not important and it involves selection of team, forming geometrical figures, distribution of things etc. Factorial = Factorial are defined for natural numbers, not for negative numbers. n! = n.(n-1).(n-2).........3.2.1 For example: 1) 4! = 126.96.36.199 = 24