The enemies of Pac-Man are known as ghosts or monsters. Their movements are actually programed into the game (even though they seem random) for example: the orange ghost "Clyde" actually moves toward the lower left of the game board if Pac-Man is facing a certain way. They are programed this way so that the game doesn't become boring. 

here are what the other ghosts do in the game:

the red ghost "Blinky" or "Shadow" is a chaser
the pink ghost "Pinky" or "Speedy" is an ambusher
the cyan ghost "Inky" or "Bashful" is very fickle
and the orange ghost "Clyde" or "Pokey" is said to be stupid and slow.

info and picture from: http://en.wikipedia.org/wiki/Pac-Man