...That's a rather difficult thing to do, but I'll try my best. XD;
It is, as you may have expected, a story about pirates roaming the sea. It follows the story of Monkey D. Luffy and his pirate crew, the Straw Hat Pirates, as they strive to find the world's greatest treasure, the One Piece. There are plenty of enemies they made along the way, all the way from a pirate crew led by a clown to members of the Shichibukai, the seven unbelievably strong Pirates who work for the World Government. I'd say more, but then I'd spoil the story for you. ;D
I hope that made sense, haha.