Definition: Perforations are the holes that are inserted in between stamps. They are placed there to make the stamps easier to separate. Early stamps did not always have perforations, and sometimes they are not always added to the stamps. Such errors can sometimes be valuable.
Sometimes a stamp issue will have different perforations, if they are produced at different places for example. This means that one stamp will have different varieties and knowing which is which will be important for the collector.
A perforation gauge is the tool stamp collectors use to measure perforations.