A song is a sequence of notes that form a melody. It can be written in any number of ways and can have any structure that the composer desires. It can also be performed in many different ways such as with an instrument or vocals.
A chorus is the big payoff and climax of a song. It is the part that tells a story and evokes emotion in the listener. It usually recurs multiple times and contains the name of the song or a repeated sentiment such as “let it be.”
The verse is the main narrative section of a song that describes events, images and emotions that are important to the story. It is lyrically different from the chorus and usually has a unique set of lyrics that are used only in this section. Musically, it is a chord progression that usually recurs and builds upon the tonic or home key of the song. It may be a single or multiple chords that are reminiscent of the tonic, such as the iii and vi chord in C Major.
The bridge is a break from the repetitive nature of the verse and chorus. It often has different lyrics from the verse and chorus and is a chance for the musician to experiment with harmonies. It is also a time to build momentum and anticipation towards the chorus.