Luffy is a laid back, stupid guy who seems to annoy everyone he happens to meet. But he also has a good heart and fights against bad guys if he has a reason (for example saving somebody's life et cetera).
Luffy is unbelievable strong and it seems nothing , except drowning, can hurt him. He has eaten one of the devil's berries, the gum gum berry which makes his body rubber but as a cost he will never learn to swim, which is a bad feature for a pirate.
Luffy grew up in a small town, which was a base for a group of pirates when he was little. Those pirates were nice and laid-back and weren't easily angered. But if anyone would hurt their friends they were ready for action. Those pirates became Luffy's idols and so he decided to become a pirate king, the greatest pirate amongst all, and go search for the onepiece treasure which is believed to give its owner fame, money and so on.
Onepiece is a the greatest treasure among the seas and all the pirates are looking for it. On his journeys Luffy notices not all pirates are nice and fair and unlike him, everyone aren't fan of pirates.
The art style isn't really my taste and that's the reason why Iven't given this popular series a try until now. But the plot is good enough to make up for it. The story is a good mix of humour and drama. The series is also adventurous and in every book thus far they face a new opponent. And the background stories explain well why the characters react to things as they do.
For example Nami has grown a strong hatred towards pirates because they have killed someone dear to her and they always destroy everything.