They are the ways to count how many outcomes in various situations.

Combinations:

Permutations:

Passwords:

for a 4 digit password and 0,1,2,3,4,5,6,7,8,9 

permutations: If order matters and no repetition: 5040 outcomes

combinations: If order doesn't matter and no repetition: 210 outcomes

possibility: repetition allowed: n^r= 10000 outcomes