The Muppets are the legendary band of Marionette puppet (from which the term "Muppet" derived) characters, many of whom made the slow progression from commercials, to television, into full-fledged movie stars. Individually, Muppets are made by Jim Henson or his company's workshop. Although the term is often used to refer to any puppet that resembles the distinct style, the term is a legal trademark linked to the characters created by the Jim Henson Workshop.
The earliest appearances of Muppets were in mid-1950s commercials, namely Kermit The Frog and Rowlf The Dog, but Jim Henson started working on Seseme Street in the late-60s, and the Muppets slowly became a television staple. After a small cast of Muppets from "The Land of Gorch" gained adult-oriented exposure with poor writing/execution on Saturday Night Live, the Muppets were widely seen as strictly children's entertianment in the United States, so Jim Henson took the Muppets overseas in 1976 when Lew Grade offered deal where Henson would produce his show in England for stations in United Kingdom with distribution rights enabling the program to air in the United States and over the world.
"The Muppet Show" aired from 1976 through 1981, and its popuarlity warranted a trilogy of silver screen appearances with The Muppet Movie in 1979, Great Muppet Caper in 1981, and Muppets Take Manhattan (1984), the latter of which was released after the show had gone off the air. Jim Henson Workshop landed another program called Fraggle Rock in 1983, which aired on HBO, as well as a fully animated series called Muppet Babies which started in 1984 and aired on CBS until after Jim Henson's death on May 16, 1990.
In addition to Jim Henson Workshop where the Muppets came to life, there was Jim Henson Creature Workshop, which was frequently licensed for films, such as The Dark Crystal, Labyrinth, Teenage Mutant Ninja Turtles: The Movie and its sequels, and (more recently) Where The Wild Things Are.
For more information, please visit the Muppet Wikia.