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. Again, I will say that songs do not have to rhyme, but I will also say that the vast majority of successful songs do rhyme and would strongly suggest you consider using this wonderful tool to aid your songs. Take caution though because clich ', boring, predictable rhymes can ruin any song.