What is AF_INET, and why do I need it?

I’m getting started on socket programming, and I keep seeing this AF_INET.

Yet, I’ve never seen anything else used in its place. My lecturers are not that helpful and just say “You just need it”.

So my questions:

• What is the purpose of AF_INET?
• Is anything else ever used instead of it?
  • If not, why is it there? For possible changes in the future?
  • If so, what and why?