Pac-Dots, also known as Pellets, are easily the most common item in the Pac-Man games. In the arcade version, Pac-Dots are the small white dots, scattered about the stage. In the 3-D games, they are rounder in shape, & are now yellow. Pac-Man is able to eat many of these at a time.
In Pac-Man World, Pac-Dots are used as a currency of some sort. At the end of each stage, the Pac-Dots that Pac-Man collected will be thrown into a machine which counts them and gives the player points. Also, only in this game, Pac-Man can shoot the Pac-Dots and Super Pac-Dots (only after charging) from his hand.
Power Pellets are the bigger, blinking Pac-Dots that turn ghosts blue. While the ghosts are temporarily blue, the Power Pellet allows him the ability to eat the ghosts.
Fly dots are found in the 3-D games. They first appeared in Pac-Man World and are a chain of Pac-Dots with the first in the chain having a red arrow pointing outwards. If Pac-Man swallows this first dot, he will fly through the chain while eating each dot on the way. The Fly Dots usually lead Pac-Man from one place to another, which was unreachable before.