Skeletor is, as the Sorceress of Castle Grayskull calls him, a demon from another dimension, Infinita. He is a blue-skinned, skull-faced warlord who rules the dark side of Eternia from Snake Mountain with an iron fist.
His title is Evil Lord of Destruction. He learned black magic from Hordak, the powerful warlord of Eternia's sister planet Etheria, of whom he was once the second-in-command. In the animated movie "The Secret of the Sword", it is revealed that Skeletor betrayed Hordak, who had abducted Princess Adora after his forces were defeated, by telling The Sorceress and Man-At-Arms the way to the Hordak's secret base.
Skeletor's betrayal of Hordak is fitting, as it is revealed that Skeletor himself built Snake Mountain (which in the 2002 series was portrayed as originally being King Hiss's base of operations).
Skeletor's portrayal in the series is generally comical, as the show's writers were forced to use villains mainly for comic value to keep the show suitable for young children. While he had a genuinely sinister personality in the beginning, as the series progresses Skeletor is treated as more and more incompetent, often bordering on a pantomime villain. However, several episodes still show the greater extent of Skeletor's evil, such as "The Problem With Power" in which he tricks He-Man into thinking he has killed a man with his own power, knowing that He-Man will exile himself in shame for his mistake.