They often have 36 to 48 pre-cut squares of coordinating fabric. They make putting a quilt top together so easy and fast! Love 'em! (Just a note) When you buy a charm pack be sure to get extra coordinating fabric for borders, backing, and binding. I've made the mistake many times to think "oh, I'll find some later" but it is hard to find just the right fabric later! Have fun sewing!!
I'm going to try and explain how to make an easy quilt top out of "charm packs".
Now take that stitched and pressed 9 patch and cut it in half,
like so...
And then take each half and cut it in half again, like so. You will have 4 of these for each 9 patch.
You can then move them around in different ways,
add sashing or a border..what ever makes your day! You can make a table runner, or baby quilt or full size quilt depending on how many charm packs you use. I had a bunch of charm packs and didn't know what to do with them, this has helped me use them up~it's quick and easy!
This is a view of the National Historic Reserve that is just across the street from our apartment. I wish I had a better camera so you could see all the little blue flowers that are blooming amongst the dandelions~they were beginning to mow today so I thought I better get a shot.
Deacon Charles Robinson enjoying his ice cream like only a child can! 
The Robinson guys: Grandpa Great, Gramps, Draper, Olson and Deacon
The beginnings of a baby quilt.