Preparing for this trip, I brought along three lenses. I usually travel with one, but this time I had my 18-200 zoom, my new 10-24 wide angle and a new F1.4 prime. A prime is fixed-focus lens, meaning it has no zoom. It has a very large aperture (opening), that allows it to take great photos in very low light and focus quickly. Prime lenses are typically used for portraits, and when dialed all the way to F1.4, they have a very narrow depth of field. For example, you might focus on someone's nose, but their eyes would be out of focus.
In the shot above, I focused on one paragraph of the page, which is sharply in focus while the rest of the photo is somewhat soft. This is a great way to add emphasis in your photo, or to create those artistic, soft focus effects.
Every amateur photographer with a (D)SLR should have a prime lens. You'll learn to "zoom with your feet", and you'll be amazed by the great photos you can take. Further, you leave your flash at home.