Spring Loaded Camming Devices (or SLCD for short) usually consist of three of four cams mounted on an axle. They are designed so that once placed into a crack in the rock face, any force placed on pulling the SLCD (i.e. a fall) will force the cams to spread open further, essentially holding the climber in place.
SLCD’s have a trigger mechanism, which when pulled with a couple of fingers, closes the cams together and aligns them so that it can be placed into the crack easily. SLCD’s, like all other climbing protection, are made by a number of manufacturers all of whom give their SLCD’s a different name. Some of the most common include Friends, Camalots and Aliens.
A diagram detailing how a loaded cam works.
As shown above, the more pressure placed on the cam, the wider it will attempt to open.