He-Man and the Masters of the Universe is a cartoon that is set in a sword and sorcery planet called Eternia. Eternia is ruled by King Randor; his son is Prince Adam, the chief character of the stories. Prince Adam is a cowardly blond muscleman dressed in a Cote d'Azur chemise. However, he possesses a magic sword, and when he holds it aloft and says the magic words "By the Power of Grayskull; I Have the Power!" he is transformed into He-Man, the Most Powerful Man in the Universe. He-Man is a brave blond muscleman in a skimpy, ineffectual looking baldric. Prince Adam's pet is a cowardly green tiger named Cringer; but when Prince Adam changes to He-Man, Cringer becomes a giant green tiger named Battle-Cat, who has a saddle He-Man can ride in, and more useful looking barding.
The Power of Grayskull mentioned above has its seat in the aptly named Castle Grayskull, the entrance to which is indeed a big grey skull. A Sorceress resides therein; she is part bird and part woman, and possesses the talent of changing herself into a falcon. The Sorceress is among the most powerful magic-wielders on Eternia, and she also has limited use of the nigh-immeasurable power held within Castle Grayskull; but only for the purpose of its protection. She also communicates telepathically with He-Man, and apparently was the person responsible for showing Prince Adam the knack of changing into He-Man.
He-Man has other allies, many of whom are in the King's service. There is Teela, Captain of the Royal Guard, who often assists He-Man in his battles. Her father Duncan is known simply by his title Man-At-Arms; he invents weapons and other useful devices for the King. He-Man's friends also include Orko, an inept magician from another dimension. Orko can levitate; that's how he gets around. When Orko is just showing off around the palace, all of his magic usually goes awry; but when push comes to shove, he can usually pull through. There is also Ram Man, whose head is useful for knocking things down, and Stratos, who flies. He-Man also has a sister, named She-Ra, who only occasionally appears in the series but was given a series of her own. She-Ra was introduced in the feature length animated film He-Man and She-Ra: The Secret of the Sword. She-Ra's own show ran from 1985 to 1986.
He-Man's chief adversary is Skeletor. Though his origin is mysterious, some believe Skeletor to be Keldor... King Randor's younger brother. Skeletor is a blue muscleman with a skull for a head, wearing a cowl. Skilled in dark magic as well as all forms of combat; Skeletor's weapon of choice is his Havok Staff: a ram's skull atop a large rod which serves to channel his magic and amplify his powers. Skeletor's base of operations is Snake Mountain, a carven fortress made of polished black basalt, which has a giant stone snake coiled around it.
Skeletor's allies include Beast-Man, who's hairy all over and can control nonsentient lifeforms; Evil-Lyn, a powerful witch with the usual complement of malevolent powers; Whiplash, whose tail is a formidable weapon; Mer-Man, half human and half fish, who controls sealife; TriKlops, a 3-eyed mechanic and inventor; Clawful, a giant anthropomorphic crab; Stinkor, an anthropomorphic skunk with magical control over his own stench (he only appeared in the written comics and 2002 animated version, never the Filmation animation); and Trap-Jaw, a weapons expert whose metal jaw can bite through anything.
The leitmotif of the episodes is Skeletor's repeated attempts and repeated failures to gain admission into Castle Grayskull. Apparently, were he to succeed, he would be able to wrest the power of Grayskull from He-Man, and conquer Eternia. He-Man, not surprisingly, resists these attempts.
Despite the limited animation techniques that were used to produce the series, the cartoon was remarkable for breaking the boundaries of censorship that had severely restricted the narrative scope of children's TV programming in the 1970s. In the 1970s, it was noteworthy to at last see a cartoon with a muscular superhero who was actually allowed to hit people. The cartoon was controversial in that it was produced in connection with marketing a line of toys; advertising to children was itself controversial during this period.
In 1987 a film was made of He-Man, under the title "Masters of the Universe", with Dolph Lundgren in the title role, Frank Langella as Skeletor and Courteney Cox in an early supporting role. The IMDB entry for the film can be found here.
In 2002, the series has also been revived successfully and can be seen on the Cartoon Network. The action figures are noteworthy, having been sculpted by the Four Horsemen.