A hook is a musical or lyrical phrase that engages your ear, and it's the most important element of any pop song worth its salt. A good hook is catchy - the German word for it is ohrwurm, which translates as 'ear-worm'.
For a pop song to be truly firing on all cylinders, there must be a cluster of hooks all working together to imprint the song well and truly into your memory so that when it finishes you want to play it again, and when you can't play it again you hear it in your head anyway.
To put it simply, a hook should be short and simple - that way it's easy to remember, whether you want to remember it or not. If your song is successful, the majority of people who hear it are not going to be lyricists or musicians, and your hooks must transcend the business of writing and be completely accessible to all. They must be instantly singable, by anyone.
Some examples classic hooks are "Woo hoo" from Song 2 by Blur; "Money, money, money" from Price Tag by Jessie J - or from Money Money Money by ABBA; "Na na na, na na na-na na" from Can't Get You Out of My Head by Kylie; or the often-cited opening guitar riff from the Rolling Stones' Satisfaction.
Don't make the mistake of thinking that a simple hook is moronic - many times they work for quite complex reasons. Blur's "Woo hoo" acts as the glue to help disguise and turn around the monster bass and guitar riff happening underneath it. The Satisfaction riff creates a plagal cadence - a musical device for the creation and release of musical tension with its roots in ancient choral music, but in this case the context is changed to rock 'n' roll.
Lurking just beneath the skin of every successful song are other kinds of hooks, some longer and more complex. The inclusion of shorter, more obvious hooks keeps you interested long enough for the big hooks to sink in, and once they're there, they're there for good. Visualising a big fishing hook, bristling with bait and other smaller hooks attached to it, is a good way of keeping in mind how you have to ensnare your listener. Whichever way they swim, there can be no escape!
Tips for making a strong hook
Simplify until a hook appears
Take an objective look at an existing lyric in your track. Get rid of any unnecessary words. Ask yourself: "What am I trying to say? Will someone understand what I mean? Is it memorable or annoying?" Keep going back and distilling - this gets easier with practice - until a hook emerges.
Write for a child
Imagine you're targeting a five-year-old to ensure effective communication of your concept. You can still write on multiple levels; simple does not equal babyish! Use as few notes as possible and employ any sound or word (real or made up) that creates something the mind can latch on to. In cultural anthropology, hooks are sometimes known as memes or behavioural viruses - think "Talk to the hand" or "Wassaaaapp" from the Budweiser TV ads.
To continue the hook/fishing analogy, hooks alone don't make the most successful songs - you need the line, reel and rod to land your fish! Hooks must be anticipated - the listener needs the hooks to be delivered at the right moment in the context of the song. Like a joke's punchline, the hook needs to exist effortlessly within the song's space, working in harmony with the other elements that surround it.
An effective hook needs to be repeated. The same hook can be replayed in different keys to create a melody. Kylie's Can't Get You Out of My Head uses "Na na na" as the main hook, but padded out with extra "na na na-na na"s. The main "Na na na" happens exactly the same way every time the chord changes.
Structural hooks help you guess what's coming by using different patterns of repetition. Like internal rhyming, a hooky melody or lyric can be made up of smaller sub-hooks. Dramatic changes of pace/style are also effective, such as the stop-restart middle eight of Dexy's Midnight Runners' Come on Eileen, or the time signature changes in Kasabian's Fire.
Great records can benefit from sonic production hooks too, and if you're a writer/producer you should definitely include some of these in your 'demo' production of the song (which, needless to say, should be of broadcast/ release standard). Think of the radio vocal sound of Video Killed the Radio Star by Buggles or the operatic middle eight of Queen's Bohemian Rhapsody - or try to imagine Cher's Believe without the Auto-Tune.
A storyline hook involves creating one huge over-arching hook to send the song in a different direction just before the last chorus. Listen to Stan by Eminem, Babooshka by Kate Bush or Rupert Holmes' Escape (the "pina colada" song). In these genius songs, the chorus is the same throughout but the context changes at the end to provide a final plot twist.
Steal someone else's hooks!
Hip-hop established the magpie culture of adopting parts of other recordings as part of a new collage of sound. Often MCs rap the verses over hooky sampled beats and breaks and use an entire chorus from another tune. It's a bit like rent-a-hook, because you have to pay for it, but it's a great way to complete your tunes if you only seem to be able to write verses!
Create a new sound
Supergroup ABBA not only had the traditional melody/lyric hook thing sewn up, but also had a flair for creating original sounds in the studio. Using clever orchestration of different instrument combinations, they devised some classic intros. The keyboard intro to Man After Midnight was sampled by Madonna recently, and check out Money Money Money and Take a Chance on Me for other examples.
For all the advice that the recording singer-songwriter needs in 2012, check out Computer Music Special 52 - the Singer-Songwriter Production Guide - which is on sale now.