 Congruence class

The congruence class of $a$ modulo a positive integer $N$ is $\{a+Nk:k\in \mathbb{Z}\}$. Two integers have the same class exactly when their difference is divisible by $N$. Addition and multiplication of representatives descend to well-defined operations on $\mathbb{Z}/N\mathbb{Z}$, because replacing a representative by a multiple-of-$N$ translate changes the result by a multiple of $N$.