A jelly roll-(42) 2 1/2 inch strips of one color or one fabric line, originated by Moda fabrics
Charm- a charm is an individually cut piece of fabric, they come in 4 inch packs, and 5 inch packs. A charm pack will be 40 or so 5inch or 4inch squares.
Fat Quarter - a quarter yard of fabric cut the "fat" way, 18 inches by 22 inches instead of 9 inches by 44 inches.
All of these are used when you want a small piece of fabric, not all of the same yardage or pattern.
If you go to the Moda website, they give free patterns using these "pre-cuts".