There is a good summary here: http://www.enotes.com/sniper/46368

But briefly, it is a story of a sniper on the rooftop with another sniper trying to kill him. Its a kill or be killed situation. We see the sniper doing everything from the mundane (drinking whiskey, eating a sandwhich) to the extreme (killing an old woman, being shot) He finds out at the end that he has just killed his brother

A detailed summary is available here: http://www.enotes.com/sniper/46368

However, if you want the story in a nutshell, this story is set in Ireland, in 1922, when the country was divided, fighting a civil war. It follows a sniper who sets up for a kill, gets shot at, and then kills the guy who shot him, only to find out it was his brother.