This story is about pirates, which in this manga translates into "badasses", who sail around on the ocean all the time except when fighting with others. The way it works is that you start with the weakest and then you continue moving East until you have defeated everyone and then you become Pirate King.
This story is also about friendship, you will recognize a Nakama when you meet them because they will have a Dream and they will fit in with your group's theme, so if you are a clown you are going to collect lions and trapeze artists and if your name is Cat you will be required to hang out with people who can normally be found at anime cons.
Mostly though this story is about huge Who Would Win
Getting a profession works much like multiclassing in D&D, so if you have studied cooking or history you will become a fighter-chef or a fighter-archeologist, if you are a pure fighter you'll have twice the Training From Hell
and Heroic Spirit
Alternatively you can just eat a Devil Fruit which is an instant power up and you don't have to train at all. All Devil Fruits belong to a certain type, from "pretty useless" through "awesome" to "really friggin' awesome". The only risk is catching a really pussy ability but Devil Fruits also level up so after a few arcs you can come back and proceed to kick everyone's asses.
These guys are a joke, but they still oppress the part of the population that do not want to be pirates and live free. The only real risk is if they are an Admiral or a Vice Admiral because they also rank after badassery rather than military training.
These guys are so badass that even the Marines pay them to leave them alone. Shichibukai translates into "evil pirate" so you don't want to be one of these guys because you're not evil. It's still worth extra points if you beat them, and you will practically have to except for the ones that have turned good.
Yonkou stands for Plot Device
, these guys are uber-pirates who can crush anyone & appear anywhere and no-one will see their fleet sneaking up on them. Plus they call their Nakama "division commanders" which sounds a lot better than "sniper-liar-handyman" or "monster-doctor-reindeer".
So basically this manga is a good guide if you want to become a real man like myself.