The four ghosts from pac-man were:

Blinky (red)
Pinky, (pink)
Inky, (blue)
Clyde, (orange)

It is the fact that each of these ghosts exhibits a distinct personality that makes this game so entertaining and playable. Once acquainted with their personalities, it is possible to predict their movements.

Here they are:

Blinky - Chases. Will usually take the shortest route to you, and tends to follow.
Pinky - Ambushes. Tends to take a more roundabout way to pac-man. Deadly.
Inky - Freak. This dude acts strangely. He moves about the board fairly randomly, but sometimes chases when he gets in close.
Clyde - Idiot. Moves randomly. Not much of a threat.

N.B.: The blinky-pinky combination is the most menacing. Watch out for them in the corners.
Next time you play some pac-man, notice that it's usually these 2 that get ya.

These personalities stayed the same through Ms. Pac-Man and Pac Man Jr. Clyde went through two name changes, though, called Sue and Tim in the following two games, respectively.