(UK) Member of the British Royal Marine Commandos.
(USA) Member of the US Army Special Forces.
"Green Beret" is the nicknames, because of their headgear. Which is a green beret, though this is moreso on the US Special Forces, than the Royal Marines.
Don't mess with either of those fucking elites though. The green berets = 10 seconds from pwnage (if you're a terrorist)
Yank: I was in the green berets, dude.
Brit: I was a green beret, mate.