The chorus is a section of lines that generally contain the catchiest part of the song. Usually the chorus contains a songs hook. A hook is a phrase of words or music that catches the listeners ear and if the listener remembers anything of the song, it's usually that part. The hook is often the title of the song and is similar to a slogan for a company. In most cases, a song contains a chorus that is the same or has only very small changes to it's content each time it's repeated.
Some songs have no chorus, but most do.
A song format of AAA would mean three verses with no chorus for instance. Some songs use a bridge as well. A bridge is usually of different length than a verse and usually has different music accompaniment. There are guidelines or principals though that we will continue discussing here. Below I will go into some detail of rhyming in songs.