Lower laundry bulk, quick drying, customizable absorbency, excellent value and unmatched durability.  The trade off is a bit of extra effort at diaper time. 

Prefolds are piece of cloth that you fold a few times and place in a cover.  They are a good way to get a few uses out of a cover and a good option to use with a newborn or a smaller baby.  Another option with prefolds is to use pins or a Snappi device to keep them closed before putting the cover on them.  These can tend to lower overall cost since you only need a few covers and the prefolds tend to be less expensive than an all in one diaper.  Some prefer the easier care, lower bulk in the laundry bin, and long life of prefolds, this is why these are popular with diaper services.