Stray (abandoned) and feral (unsocialized) cats are an enormous problem in almost every community. The following factors compound this situation:
As inconceivable as the size of this problem is, it would not exist at all if every house cats were spayed or neutered. Every homeless cat can be traced back to an unsterilized house cat. It has become routine for people to move to another home, leaving their intact cat to fend for herself, brave the streets, predators, the harsh seasons, and disease, and produce kitten after starving kitten. These kittens, if they survive, grow up completely unsocialized, or "feral". These feral cats continually reproduce at an astonishing rate, producing litter upon litter of starving, weak, and ultimately doomed kittens.
There IS a solution! Trap-Neuter-Return-Maintain (TNRM) programs exist all over the world. These are people like you working independently or in groups to humanely break this birth cycle.
It is quite simple: using a specially designed humane trap a cat is captured and transported unharmed to a veterinary clinic where s/he is anesthetized, surgically sterilized, and vaccinated. Once s/he is sufficiently recovered (1-5 days, depending on the technique, anesthetic, and the cat's gender) they are returned to their home colony, where a volunteer or group of volunteers regularly provides food and water. Further trapping in that colony may be required. For instance, if the aged die off and younger, unsterilized cats capable of reproducing are allowed in.