Lets see:
A Charm - a 5x5 inch square
A Jelly Roll - a roll of strips 2.5 inches x WOF (Width of Fabric, usually 44 inches). There are usually 40 in a roll.
A Fat Quarter - a piece of fabric that measures about 18x21/22 inches.
Each one can be used in different quilting patterns. The Charms are great if you want to have a piece of each fabric in a line.
There are books that have been written specifically for each type of project, but the uses for each aren't limited.