Background
-> the original show
Grease started life as a play with incidental music, opening in Chicago in 1971. Reworked, it opened as a musical in New York in 1972, and was an immediate success. It was presented in several theatres on & off Broadway, & ran for a total of 3,388 performances.
The London production started in 1973, starring an unknown Richard Gere. Later Neighbours actor Craig McLachlan played Danny.
-> the movie
The film version came out in 1978, with many new songs (including the disco opening track), & starring John Travolta & Aussie singer Olivia Newton-John. To many people these are the "real" Danny & Sandy.
Several songs were written just for Olivia (including Hopelessly Devoted to You & You're the One that I Want) & became big hits. Olivia was known as a singer, & became a superstar through Grease.
John Travolta was more famous as an dancer/actor, particularly in Saturday Night Fever. Grease made him famous as a singer too.
A sequel, Grease 2, was made, but without any of the big names. It was not well-received and failed to make much money.
The movie slogan was "Grease is the word", from the opening song. It was re-released as a 20th anniversary DVD, with the slogan, "Grease is still the word".