The more holes per inch, the easier the stamps separate. However, the PO doesn't want the stamps falling apart when they are handled. The result of these two somewhat conflicting requirements is the reason that some issues have many different perforations. Perforations are one way we, as collectors, classify different issues of the same stamp. Measuring perforations are one of the easier things to deal with when learning to collect.
Perforations come in three basic types.
There is the classic perforation where paper has been removed creating a line of holes to ease the separation of the stamps. Most perforations are of the classic, round-hole variety with paper removed. There is one form of perforation where the holes are rectanglar and paper is removed. These perforations are a line of little rectangles. When you have two stamps attached to each other, the rectangles are very clearly different from a hyphen roulette, which is just a series of dash-shaped cuts.