In this particular case, with Cam we mean a specific method how movie pirates create illegal bootleg copies of original movies. Cam method is basically just a guy who uses either regular or professional camcorder to shoot the movie either from TV or actually inside a movie theatre (in some Cam copies of movies you can actually see audience as well :-). These bootleg copies are normally then encoded into VCD, SVCD or DivX format and distributed illegally over the Internet.
Recently however, there have been movements to ban camming from theaters, and to put the offenders in jail or leave them with large fines.