I'M LAZY, SO I JUST COPIED & MODIFIED WIKIPEDIA'S ENTRY.
Trap-neuter-return (TNR) is a program through which feral cats (not belonging to particular humans and living outdoors and not liking humans) are humanely trapped; sterilized and medically treated; and returned to the outdoor locations where they were found. If those locations are deemed unsafe or otherwise inappropriate, feral cats (unsocialized to humans) are relocated to farmyard homes. Kittens still young enough to be socialized and friendly adult cats are typically placed in foster care for eventual adoption into homes as companion animals rather than returned to the outdoors.
If you want to find TNR programs in your state, visit GET YOUR FIX.