Structure of a Song:
THE VERSE: We all know that a verse is the part of the song which tells the story. Most songs have no more than four verses, which would include repeating the first verse at the end. Bob Dylan has written songs with dozens of verses, but none of those ever became hits. Of course, you can get away with only one verse repeated over and over again, if you want. The Red Hot Chilli Peppers, with 'Roller Coaster of Love', and Nirvana, with 'Something in the Way', are two that did.
THE CHORUS: The chorus is the part of the song which you want people to be singing along with by the end of the song -- the first time they hear it. One easy, effective and sure-fire killer way of making a chorus lift to maximum hit-ability is to find the highest root note string sound you can and have it simply playing all the way through. It sounds corny, but just try it. It could be one of the elements that makes your track a worldwide smash hit. Ask the Pet Shop Boys what they think of this idea.
THE BRIDGE OR TAG: This is a section that links the verse and the chorus together. That music shop favourite 'Wonderwall', by the mighty Oasis, has a perfect example of a bridge, if a little long and unadventurously used ("And all the roads we have to walk are winding..."). The song also has the 'two verses at the beginning' trick (see next section).
THE MIDDLE EIGHT (or, as James Brown would shout, "Take it to the bridge") is a third melodic part, usually placed after the second chorus to break up the song pattern. It's called a middle eight because it's usually eight bars long, but there's no law saying it has to be that length or even there in the first place -- whatever feels good and fits the bill. No-one has ever done a study on this but I would hazard a guess that 50% of records have a middle eight, and of those, 50% are eight bars long. Michael Jackson used this device for effect in 'Billie Jean' ("People always told me, be careful what you do..." -- which, by the way, is eight bars long).
A KEY CHANGE: Why? Because it can lift a song at that difficult 'two-thirds of the way through' stage, where the listener's interest is beginning to waver. The usual key change is to move up a tone (from A to B, for example). It's advised, for maximum effect, to build into this with a huge drum break or a dramatic pause. Key changes down are seldom, if ever, used, because they give the opposite effect of uplift. And note that more than one key change per song can be more annoying than exciting. There's a classic example of a key change in the Whitney Houston hit 'I Will Always Love You'.
THE CODA is a cool way of ending a track. It's either the chorus hook repeated continuously, or a new section used to tail off a track. One of the most exciting codas used in popular music is the end of Elvis Costello's 'Accidents Will Happen' -- the bit that repeats the words "I Know", ad infinitum.