GunBound is a (currently free) online multiplayer game. If you've played older games like Scorched Earth, or more recently any of the Worms incarnations, you'll be fairly familiar already with how GunBound plays.
Basically, you select a bot and fire at other bots by configuring your angle and power. That's the ultra-simplified version.
To learn more about the game, check out Gunbound-HQ, a fairly comprehensive destination for GunBound things. (top)
GunBound is offered by Softnyx, on their website at gunbound.net. Please note that it's .net, not .com. gunbound.com leads to the Korean version.
Once there, go click Download in the upper right hand corner, follow directions, check your system requirements, and get it. Note: it *is* a Windows-only game. (top)
Again, go to the main GunBound website (gunbound.net). Look near the upper-left hand corner and you should see what you're looking for - click the button "Registration" and go from there. Note that currently logging *out* of the GB website is currently broken, so keep that in mind. (top)
It goes down all the time for reasons they may or may not tell us. Nothing you can do. (top)
If I ever hear this question from anyone, I am immediately going to direct them to here and here. I don't enjoy catering to laziness any more, so do yourself a favor and read up on what goes on. (top)