Here's an excerpt from this Craftsman's Corner page that reinforces what Ebbie just said (http://www.cqkmusic.com/CCorner/a13.htm):

THE REALITIES OF RADIOPLAY

Capturing the Listener

....In previous generations the hook has been called a motif or a refrain, but the concept is crucial for any kind of effective musical communication. (George Frederic Handel may never have heard the term, hook, but he definitely understood the principle. Could anyone write a more effective hook than "Hal-le-lu-jah" ?)

The dictionary definition for the implement known as a "hook" is this: A tool which is used to catch something, hold something, sustain it and pull it along. This is also a very good definition for a songwriting hook. A well-crafted hook will catch the listener's ear despite all competing visual and audio distractions. It will hold the listener's attention….sustain listener interest and pull the listener along to the very end of the song. And in case you hadn't noticed, all of these functions accomplish what radio hopes to achieve -- namely keeping their audience from changing channels.

Of course, the subject of hooks could fill several articles all by itself. In future editions of The Craftsman's Corner, we will definitely explore it further. But for our purposes here the following considerations are essential:

* The hook is the most memorable musical and lyrical element of the song. It is the thesis statement -- the one-phrase summary of the whole creation. It is like a diamond on velvet. The rest of the song is the velvet background which displays the beauty of that one magic "diamond phrase" known as the hook.
* The hook is usually also the title and should be strategically placed at the beginning or ending of the chorus….or at the first or last line of the A sections in an AAA or AABA song ….where the listener expects to hear it.
* A well-written hook/title will be so clear and obvious that listeners will know it from the first hearing and will be able to request the song BY TITLE when they call the radio station to request it.