Just a Question

I have been playing around with GIMP Version 2.2 for a while now, but I have never figured out or bothered to figure out what plug-ins are.

So the question is..........What are plug-ins, what are they for, were do you get them and how do you use them?

Yes I know that was a 4-in-1 question, but any answers are appreciated.

Thanks in advance.

Also posted in gimp_users