probability reference   (link)

http://www.saliu.com/permutations.html
Any finite number of elements can be put together in groups based on certain rules. Such groups are known as sets. The sets can comprise from 0 elements to infinity. There are four types of sets, from the most inclusive to the least: exponents, permutations, arrangements, and combinations. The number sets are the most important mathematically. We can substitute the numbers by alphanumerical elements, such as words, names, any strings of characters. In the case of the alphanumerical sets, mathematics works with the indices, indexes of the respective elements.

An example of exponents (N=3, M=3): 111,112,113,121,122,123,131,132, etc. (a total of 27 sets).
An example of permutations (for N = 3): 1 2 3, 1 3 2, 2 1 3, 2 3 1, 3 1 2, 3 2 1 (6 elements: 1* 2 * 3)).
An example of arrangements (for N = 3, M = 2): 1 2, 1 3, 2 1, 2 3, 3 1, 3 2. (6 elements in set: 3 * 2).
An example of combinations (for N = 3, M = 2): 1 2, 1 3, 2 3 (3 elements: 3 * 2 / 1 * 2).