jelly rolls are 2 1/2" strips by width of fabric; charms are 5" squares, layer cakes are 10" squares, different collections have a different number of pieces- some have duplicates- 2 of each fabric- some are all different- it's important to check the label. and only you can decide if it is worth it to buy them- some people love using pre-cuts some hate using them- it is a way to get a piece of a whole collection of a particular line- if you make strip quilts jelly rolls can simplify the job- if you like charm quilts- charms are a fun way to go- if you have an extensive stash it is fun to cut up your own 'collections' and put together your pieces in the sizes you will use for a future project...sometimes they are well worth it- sometimes---not so much- if you want a piece of each fabric in a collection- but only a little bit a charm pack is an easy (fairly inexpensive) way to go about it.