Intro, verse, pre-chorus, and chorus are labels for different sections of a song depending upon specific characteristics of melody and accompaniment. Most songs start out with some kind of an intro, then usually it is followed by one or two verses. After that there is sometimes a pre-chorus section which lead you into the chorus. There is frequently another section, to provide contrast for the song, and this is called the Bridge
Sections that are designated with letters of the alphabet will often be laid out in A, A, B, A to give an example. In this structure, usually the eight sections or verses and the B section is a contrasting section, in this case most likely the chorus.
