DNS Subdomains


Might be a simple question.... But... I've got no idea what the answer
could be...

In the early days, one only had a .com address space (amongst the most
popular ones). These days, there is .com(this) and .com(that) and any
kind of .(whatever) you can think of.

My question...

How does one start a .(whatever) ?? Who is actually controlling these
.(whatever) domains ?