Feral cats most often start-off as a small colony around an available food source. It provides them with group protection and it is not uncommon for co-operating females and kittens (matrilineal colonies) to develop. 

While there may be a very loose dominance hierarchy in these groups, they do not form an interdependent hierarchy as, for example, would occur in dogs.

Relationships are complex, with stronger affiliative relationships between some cats and less affiliation with others – this may in part be influenced by how related they are, age, sex etc. ~ Source: the-social-structure-of-cat-life.

What is a feral colony?
A colony is simply a group of homeless cats living together. The colony site may be a in a storm-water drain, a rural property, an industrial site, a garbage dump-site, a university campus or shopping centre — wherever food (often garbage, rodents) and shelter can be found.

How do feral colonies form?
Feral colonies form wherever homeless cats find food and shelter. A single pregnant female “stray” can turn into a colony of dozens in just two years.

Cats become sexually mature between four and five months of age and multiply at an astonishing rate. Feral females usually produce one or two litters of four to six kittens each in their first year of life and then three litters annually.

Why are there so many feral cats? 
Pet owners who fail to spay or neuter their cats and allow them to roam or who abandon unaltered pets to fend for themselves are largely responsible for the feral population.