Actually, this is one of my favourite gods in Slavic mythology! So, lets get started! :P
In Slavic mythology, Perun is the highest god of the pantheon and the god of thunder and lightning. His other attributes were the fire, mountains, the oak, iris, eagle, firmament, horses and carts, weapons (the hammer, axe (Axe of Perun) and arrow) and war. He was first associated with weapons made of stone and later with those of metal.
Like Germanic Thor, Perun is described as a rugged man with a copper beard. He rides in a chariot pulled by a goat buck and carries a mighty axe, or sometimes a hammer. The axe is hurled at evil people and spirits and will always return to his hand.
With the arrival of Christianity, various churches had a difficult time trying to overcome the worship of the old supreme deities of the Slavs. In the East, the Eastern Orthodox Church gradually managed to pass much of Perun's characteristics on to a new Christian saint, Elijah the Thunderer, based upon the Old Testament prophet Elijah, whom the Scriptures state rode a flaming chariot through heaven; this seemed a good enough approximation of the old thunder god with his fiery bolts. In the west, the Roman Catholic Church offered St. Michael the Archangel, who, as a commander of heavenly armies and vanquisher of the Devil, was also a fitting replacement for Perun. It is also possible that on a local level Perun was replaced with St. Vitus, where this saint did not, due to similarities in names, replace another important Slavic god, Svantovit; however, it is also possible that already in pagan times, the worship of Perun was challenged by a growing cult of Svetovid (Svantovit). On some levels of folklore and popular Christianity, some of Perun's characteristics were passed on to the Christian God himself.
In Slavic mythology, much like in Norse mythology, the world was represented by a sacred tree, usually an oak, whose branches and trunk represented the living world of heavens and mortals, whilst its roots represented the underworld, i.e. the realm of dead. Perun was a ruler of the living world, sky and earth, and was often symbolised by an eagle sitting on the top of the tallest branch of the tree, from which he kept watch over the entire world.
While the exact pantheon characterization differed between the Slavic tribes, Perun is generally believed to have been considered as the supreme god by the majority, or perhaps nearly all Slavs, at least towards the end of Slavic paganism. The earliest supreme god was probably Rod; it is unclear precisely how and why his worship as the head of pantheon evolved into the worship of Perun. Another candidate for supreme deity among at least some Slavs is Svarog.
Here is one video with great pics of Perun and the song is not bad. :P