Symmetry operations are operations that leave the molecule apparently unchanged. In each molecule there can be any number or none of these:

These are reflections in a symmetry planes (x, y and z).

s_{h} - Horizontal Plane (y)

perpendicular to the highest rotation axis

s_{v} - Vertical Plane (z)

parallel to the highest rotation axis

s_{d} - Diagonal (dihedral) Plane (x)

the Diagonal Plane that bisects two axes