Modular Arithmetic : 

a mathematical system with a finite set of numbers; modulo m has m different numbers (modulo 5 for example the numbers 0,1,2,3, and 4). If a number differs by multiples of m , it is said to be congruent . Let's take modulo 3 for example. The element of this set are the numbers 0,1,and 2. In this system 3 is congruent with 0; and 4 with 1; 5 with 2; 6 with 0 and so on.