What is the difference between the mechanical relay and photorelay?

A mechanical relay consists of mechanical contacts, whereas a photorelay consists of semiconductor contacts where output is composed of MOSFETs. Compared with the mechanical relay, the photorelay has the following merits:

High reliability without wear of the contacts (Long life)
Small, light-weight design for assembly
High-speed switching (5 times faster than the mechanical relay)
Low drive power consumption: several mW (100 mW or so for the mechanical relay)
Less noise with no contact