Hi! Jenn and Heidi are both right. I'd just like to add a couple of points:
A feral cat is the offspring of either already wild cats or the offspring of what was once a domestic cat (a pet) that ran away or got dumped or left behind when a family moved.
Feral cats are PRIMARILY the result of pet owners' abandonment OR simply the failure of people to spay or neuter their cats. People with pets who let them roam, if the cats aren't sterilized, are how the vicious cycle starts.
Although these figures are a bit misleading because it assumes that wild cats live seven years (when in fact they live somewhere, on average, between 1 1/2 years and three years), just one pair of breeding cats CAN, through the reproduction of all the children, produce 420,000 offspring. This costs the States in excess of $50 million annually (mostly in taxes - this does not include the many volunteer organizations and donations) in animal control agencies and shelters for cat-related expenses.
It is a terrible, terrible problem. There are many spay/neuter programs out there. One of the most well known low-cost nationwide programs is run by SpayUSA. If you are interested in help with the local kitties you take care of, The Cat Site is certainly a place to ask questions, get advice, and get help with resources.
Oh - and if you are interested in looking up low cost spay/neuter options, here's the link to spayUSA:http://www.spayusa.org/