What is Class of symmetry operations, subgroup?

Example 4.12 (Classes of $\ce{NH3}$)

In $C_{3v}$ ($h = 6$), the three reflections form one class (a $C_3$ rotation carries each plane onto another), the two rotations $C_3$ and $C_3^2$ another, and $E$ is a class alone: three classes, written $E$, $2C_3$, $3\sigma_v$. In $C_{2v}$ every operation is its own class: no operation turns one plane into the other.