When you first see a cat outdoors, you need to determine if the cat is stray (usually an abandoned or lost kitty) or feral--a truly wild cat. Feral cats will run when they see you, even if you have been feeding them for months. This distrust of humans is one reason they are able to survive in the wild. If the cat is feral, it will probably not be able to be socialized. Trapping, neutering and then releasing the cat back into its colony is the most humane option. For every 1 feral cat you see, you can be certain that there are many others. Keep close watch. An abandoned or lost cat may "talk" to you, eventually allowing you to come close and perhaps even pet it. If the cat is not feral, chances are you can trap it, find its owner or find it a good home.
For many well-meaning animal lovers, their first instinct upon spotting feral cats or kittens is to call their local animal control agency. Do not make the call. If these animals are feral, they will almost certainly be euthanized. The most humane thing to do ultimately is to try to trap, neuter and release the cats, thus allowing them to live out their natural lives, while at the same time, curtailing any future population explosions. You can easily distinguish neutered cats by the tiny "tick" or "clip" taken from their ear.
Don't go it alone. Your best bet is to contact a local feral cat organization. VIsit alleycat.org to locate an organization in your neighborhood. They will be able to lend you a trap and provide specific tips on how best to trap the cats, where to access free or low-cost spay/neuter clinics and how to best provide shelter and care for a feral cat colony. Some volunteers might even be able to foster a feral cat for a few days until he or she recovers from the surgery.
If you are caring for a feral cat colony, commit to taking responsibility for that colony. Feed the cats daily and provide them with clean water. Make sure they have shelter in inclement weather. Try to keep track of the cats and make note of any new colony members who need to be trapped/neutered/released. Chances are your neighbors will want to assist you. Ask for help in sharing responsibility for feeding the cats. Accept donations of cat food and help with trapping and transport.
Cats are incredibly adaptive and can usually survive cold winters, provided they have shelter and can stay dry. Otherwise, they can suffer from frostbite and hypothermia, just as humans can. You can build a modest shelter for feral cats for about $20 using simple materials from a hardware store. (For instructions, refer to the links in "Tips.") If this is not possible, even a cardboard box to provide shelter from rain and snow is better than nothing.