Definitive, Commemorative, Semipostal, and Airmail

The regular stamps issued in large quantities and sold sold by the post office over a long period of time
are called definitive stamps.

A stamp issued for a limited amount of time to celebrate some special event, person, or thing is called a commemorative stamp.

Stamps printed with two values are said to be semipostal stamps. One value is for the postage.
The other value is a contribution for some charitable cause or organization.

Stamps issued for use on mail which is carried by airplanes are called airmail stamps.