Jodie gives an excellent reference.
http://www.unitednotions.com/equivalent-measures.pdf
I frequently feel a bit wooly headed when around folks tossing off terms (like jelly rolls and sweetbox), causing me to stop and think about what they are trying to tell me. Although I prefer to buy yardage and cut my own, precuts have their place. Sometimes it's fun to make a little project from a pre-matched bundle (for example, Quilt-in-a-day's Buckeye Beauty wall hanging using 5" layer cake bundles). Just note: when buying a bundle pay attention to how many are included. I have found this varies from manufacturer to manufacturer and store to store.