Cams are removable devices which can be placed in rock to be used as protection during traditional lead climbing. Typically they work best in parallel cracks in solid rock. Because the camming device pushes out with 2X the force on either side of the parallel crack, it is very important the rock be solid (not a hollow flake). They are favored because they are easy to remove and are a good complement to a set of nuts or hexentrics.
Double axle cams can also sometimes be placed in a passive mode. Check the manual from your equipment manufacturer.
Placement
Look for a parallel crack in solid rock. Aim for equal camming of all lobes. The lobes should be cammed as much as possible without getting stuck. If the cam is under-cammed, then use the next size up. Beware of outwards or downwards flaring cracks, where placements are unreliable. An upward flaring crack can work, but be wary that the cam can "walk" upwards into a less secure placement through the movements of the rope. Placement should be deep in a crack, not shallow where the cam can easily pull out.
See also: