Sniper: Ghost Warrior is about an island called Isla Trueno, a democratically elected government that is toppled and some rebel fighters (which includes you) and their attempts to take back the power of the island. That, and sniping; you will, according to the developers, take things like gravity and wind into account when sniping your enemies from afar.
Other than that, Sniper: Ghost Warrior is said to offer the good old sandbox features; non-linear, do the missions your own way, etc. And you get to do more than snipe, like fight in close clombat and put up traps and other deadly activities. Me, I just want to snipe. I'm a coward that way.