On the Twelfth Day of Christmas My True Love Sent to Me...

...Twelve Drummers Drumming! I imagine the sound as urgent and climactic, rallying the listener (and in this case the reader) onwards. Drums, whether at a gig or on the field of battle, are there to energise and inspire, setting the pace for a song or a march, and pace is something extraordinarily important when you are writing fiction.
Whereas tension is linked to suspense and has to do with the facts of your narrative - discovering who  murderer is, or whether true love will win through - pace has more to do with the way in which you relate these facts.
Uniformity is the enemy of pace. There should be variety within the structure of your novel. This applies from the macro to the micro: try and make sure that your chapters, your paragraphs, your sentences, and even your individual phrases are of different lengths so that your work doesn't become predictable. If you are writing about something dramatic, keep things short; if you are in more laid-back mood you can afford to meander (but not too much). Just keeping an eye on this simple technicality will have an enormous impact on the readability of your story.
Another way of manipulating the pace of a piece of fiction is through contrast. Just as you should go slow, slow, quick, quick, slow or long, long, short, short, long in structural terms, it's important to contrast the types of scene you are writing. If you have ratcheted up the emotion during a particular sequence of your book, give your reader a break and introduce something lighter. Think Hamlet and the gravedigger scene.
Pace has elements of texture and tension and contrast within it. It's what gives your writing dynamism. It's an essential ingredient in any good book and you should always be conscious of it when you are at work.

Pacing It Out

As I am full of January sloth and seem to have ground to a halt with my re-writes this afternoon, now seems like a very good moment to talk about pace -  if I think about it, maybe I'll acquire some!

Pace has elements of structure and elements of style about it.  When you are writing, if every scene, every paragraph and every sentence are same length and move at the same tempo, your reader will very quickly start to drift off.  You can't depend on plot alone to drive your narrative forwards - as with so many things, it is not just the story you are telling, but how you are telling it which will make it stand out from the crowd.
To keep your novel pacy, you need plenty of variety: you need to alter the mood, the tension and even the language of different episodes so that you create and maintain a kind of internal dynamism. 

My hunch is that it is slightly more subliminal than simply contrasting scenes with each other - you would do this by moving from interior to exterior, from subplot to main plot, or perhaps by concentrating on a different character.  With pace it's a question of changing the atmosphere and the intensity of a scene,or even a portion of a scene.  At its simplest level, just altering the length of sentences from expansive and complex to short and snappy can help to add pace.

If you want to put this to the test, why not try writing a piece that uses a change of pace (rather than, say, a plot twist) as the forward impetus to a story, and see where that takes you?