Feral cats are un-owned, free roaming cats that avoid contact with humans. Feral cats that are picked up and taken to the local animal shelter generally end up being euthanized since the cats aren't tame enough to be placed in homes. In addition, removing the cats from their environment does not solve the problem; it merely creates a vacuum that other feral cats fill. Programs that trap, neuter, and re-release the cats back into their environment offer a humane and effective means of dealing with free-roaming cats.
If you are interested in learning more, try Alley Cat Allies