The intro: An instrumental opening for the song that introduces the tone and rhythm of the song. Certain songs, like the Beatles' "She Loves You" break typical form and open up the song with the melodic chorus.
The verse: The most common part of a song, where the main body of the lyrics and music tends to go. In Michael Jackson's "Billie Jean" and others, this is where the story is told. The "summary" of the song is reserved for the chorus.