Shaman King is an Anime/Manga about a boy called Yoh Asakura and all the things he has to go through before entering a tournament to become Shaman King. The Shaman King becomes one with the King of Spirits and is very powerful.
What is a Shaman?
A Shaman is a person who can interface with the world of gods and spirits, which allows them to do amazing things. They can ask the gods for wisdom, borrow the power of the spirits to heal the sick, and summon the ghosts of the dead into this world. Of course, by contacting the dead they can also speak with the great figures of history.
Why are there two versions?
The original Shaman King was in Japanese with English subtitles. However, 4Kids Entertainment made a 'dubbed' version which is mostly the same, but completely in English. Many fans do not like this version as many names and lines are changed completely.
So what's the storyline?
Basically, a guy called Yoh Asakura enters a Shaman Tournament (which only occurs once every 500 years). With the help of his many friends, including Horo Horo, Ren, Anna, Manta, Ryu and Faust, he has to work to the top to become stronger - strong enough to beat his evil twin, Hao. But that's a very simple description. Why don't you go any watch it for yourself?