Producer Joan Ganz Cooney and Lloyd Morrisett wanted to use television to help children from poor families get ready for school. The show borrowed the quick style of TV commercials, so an episode was "brought to you" by a letter and a number.
Big Bird, Oscar the Grouch, Bert and Ernie were there from the first season. More than 50 years later the show is still running, and local versions have been made in many countries.
Source: Wikipedia