Every
bank bigger than two buildings these days has an
ATM attached to it. Other networks, like
Cirrus and
Novus, connect most every ATM in the world to each other so that you can withdraw your own
hard-earned cash no matter where you are.
However, a couple of years ago a few banks realized that they were giving out a lot of their machines' cash to people with accounts at completely different banks. Since they had to fork over a lot of money themselves to even have the ATM installed, this was a negative return on investment for them. So they invented out-of-network fees, allowing them to charge anywhere from half a dollar up to three dollars if you're using their machine without using their bank.
They also started charging similar fees to members of their own bank for using out-of-network ATMs. Unlike the aforementioned out-of-network fees, this has no apparent justification whatsoever. Banks are greedy buggers, but then, that's what they're in business for.