Hunter X Hunter (pronounced "Hunter Hunter")
Hunter X Hunter follows the adventures of Gon Freecs and his attempt to become a hunter. No, he doesn't learn how to build campfires and kill rabbits (he knows that already).
Hunters are extraordinary people with the power to do whatever the heck they want. With their Hunter license, almost all public services become free. The license even acts as a visa, allowing them to travel to practically anywhere in the world for absolutely no cost (they can use public transport!). Not to mention the money they'd make from selling it would allow them to live 8 lives over. All sorts of other awesome perks come along with it, but I won't get into that right now.
The Hunter Exam
Basically, to become a Hunter (I'm gonna start capitalizing it), a special organization organizes the Hunter Exam. Thousands of people participate (or at least attempt to) in this exam every year. To cut the number of examinees, all sorts of diversions and traps are placed in front of them before the exam even begins.
The actual exam is different every year with different examiners, which are Hunters that are chosen by the Hunter organization. If you can complete all of the trials, you are awarded your Hunter license and are considered a professional Hunter.
The Main Characters
Gon is a wild kid. He spends most of his life on Whale Island in the wild. Because of this, he has developed exceptional abilities like being able to spot the movement of a shadow at night in a forest, and being able to understand the "language of the animals." He's basically just pure awesome in a can.
At the beginning of HxH, Gon learns that his father is not dead and, in fact, is a legendary Hunter (naturally). Gon realizes that his father left him behind so he could continue his work as a hunter. Understanding that the job of a Hunter could be so important, Gon sets off to become a Hunter and to find out what being a Hunter really means (or something like that).
Gon meets him during the Hunter Exam.
Killua becomes Gon's best friend. He's one of the few people Gon has ever met that is his age. He and Gon travel through the exam together.
His entire family is a group of assassins (Killua included). He was trained by his father and older brother, even as early as 5 years old, to be an assassin (which includes physical and mental torture). Before HxH starts, Killua (who never had a choice in the matter of becoming an assassin) breaks out and escapes from his home. He joins the Hunter Exam on a whim because he heard it was difficult and might be fun.
Age 19 (I think).
Gon meets him during the "preliminaries" of the Hunter Exam. He gets into a fight with Kurapica over formalities (Kurapica doesn't address him with a title), which ends with them becoming friends. He becomes friends with Gon and travels with him throughout the exam.
PS: the anime and manga are different. In the anime, Gon meets Leorio about 1 episode earlier (though it is pretty significant how they meet).
Leorio joined the Hunter Exam so he could avoid the fees required to take classes and exams to become a medical doctor (yes, the hunter license does pretty much make everything free). The reason for this is because his friend died from a simple illness, but the village he lived in lacked the money to pay for medicine. He wants to become a Hunter/MD so he can provide care to people in need and not ask for anything in return.
Age 17, or 18. I think it's 17.
The same with Leorio. Gon meets him during the preliminaries. Kurapica gets into a fight with Leorio after Leorio insults his clan. Kurapica becomes interested in Gon's ability to be awesome and decides to help him through the Exam.
Kurapica is the last living member of the Kurata clan. They were marked by their eyes which turned red when they got emotional. The clan was massacred by the Phantom Brigade (also called "The Spiders," due to the tattoo that all members of the brigade have. A spider) so they could steal their eyes and sell them as treasures. When a member of the Kurata clan dies in an emotional state, their eyes remain red forever.
He wants to become a Hunter so he can avenge his clan by destroying the Phantom Brigade and retrieving the eyes of his dead comrades.
The Phantom Brigade is also a major part of the series and takes up an entire arc.
Now that that's done, I'd also like to say that Hunter X Hunter is definitely my favorite shounen series ever. I haven't given it away yet, but later on something called "nen" is introduced to the series, and it will blow your mind. Among all of the shounen super powers (CHAKRA. SPIRITUAL POWER. KAMEHAMEHA), nen is the awesomest.
That is all.
PS: HxH started a year before Naruto.
PPS: HxH is created by the same guy who made Yuu Yuu Hakusho.