Feral versus Stray
I've noticed that a lot of people don't seem to realise the distinction between a "feral" cat, and a "stray" cat. This most likely results from a lot of people using the word "feral" to refer to wild behaviour; and even then, a lot of people misunderstand normal cat behaviour as being extraordinary.
Quite simply, the distinction between feral and stray usually lies with where a cat was born;
- Feral = born outside with no human influence during their early development.
- Stray = born in a domestic setting, but has lost their human connection.
Feral cats are usually born to feral or stray mothers. However, sometimes a domesticated cat will choose to have their babies away from their domestic setting, in which case, with no human interaction when young, the kittens grow up as feral cats.
Cats may become stray for many reasons. "Stray" literally means that they have strayed from their home, though that may not always be the case. Sometimes humans move on and leave their pets behind; the cats are then stray because they have no domestic setting in which they belong. There are a lot of stories of cats getting lost by jumping in people's cars, jumping on trains, being picked up by random strangers, etc. (This is why microchipping is so important). Cats can and do simply get lost, too.
Your pet cat, no matter what it's behaviour, if it lives indoors with you, is a domesticated cat. A pet.
The good news is that kittens born to feral or stray mothers can be domesticated, if they're caught early enough. Alternatively, when the mother is pregnant she may be cared for by a rescue; the rescue can domesticate the kittens, and re-release the mother once the kittens are raised, and she's been neutered so that she cannot have anymore kittens.
If you see a cat with its ear-tip missing, or a little triangle neatly cut into it, that cat (most likely) lives wild. "Ear-tipping" is performed by vets as part of TNR, when a the cat is neutered. TNR = trap, neuter, release; the method used to attempt to keep feral numbers down.
So, unless an ear-tipped cat is in distress or has itself chosen to befriend you, leave it alone as you would any other wild animal.