What is a feral cat?
A feral cat is a domestic cat which has reverted back to its wild state. It was born and has always lived in the wild. Most feral cats live in groups of cats, known as colonies or "clowders".
What's the difference between a stray cat and a feral cat?
A stray cat is a once domesticated cat who is now living in the wild, usually the result of being dumped or lost. If a stray cat produces offspring in the wild, these will become feral. Having little to no interaction with humans.
Can a feral cat be domesticated?
Most adult feral cats are too wild and distrusting of humans to be domesticated. If caught at a young age, the offspring can usually be socialised enough to happily live with humans. Some adults have been successfully tamed, but this is considerably less likely than with kittens as adults have already learned not to trust humans.
Where do feral cats live?
Feral cats can be found throughout the world, some live in the country, in woods, deserts, on farms and some in cities and towns. They survive by catching local wildlife (birds, rodents etc). Which sadly, in many cases has been disastrous for local species of fauna. It is for this reason that governments and councils have gone to great effort to eradicate feral cats, with limited success.
It is not an easy life for them. The lifespan of a feral cat is greatly reduced due to disease, injury or death from predators, humans, vehicles.
It is said that up to 400,000 feral cats live in New South Wales (Australia) alone. 
What can be done about feral cats?
This is a really difficult topic to address, with two sides. At the moment (in Australia at least) many local governments and councils are working hard to eliminate feral cats. That may be a trap, neuter, return (to the wild) (TNR), or poisoning them.
Feral cat organisations have sprung up to try and help these cats. TNR is the preferred method, and may include having people look out for these cats and provide them with food and water. There is an interesting article on TNR by the RSPCA in Australia and their take on the topic.
How can you help:
The best thing you can do is desex your pets and don't let abandon them. If you have the time or the money, contact feral cat organisations and see if there is anything you can do to help.
 NSW Government