I do charm swaps with friends. They are basically a cut piece of fabric and in a charm pack they would all be the same size.... like we swap 5" squares. You can google charm quilts and get lots of info on making them. Also there are books for using these charms.. my favorite are Nickel Quilts and More Nickel Quilts.
I am not as certain about jelly rolls... I believe they are packages of fabric that are rolled up and I think each piece is 2 1/2 " wide but I forget the length.
The fabric store or catalog should tell you the amount of fabric included in the pack or roll and that would help you compare it to a fat quarter, which is 1/4 of a yard, just cut in a different way than a true 1/4th yard.
So are you now more confused?? Barb C