One way to describe meditation is as
prayer without an
object. (ie, without praying
to anything.) This wouldn't describe all
types of meditation, but it's a good introduction. Prayer and meditation can have similar
effects.
The best description of meditation is a practical one. Try this:
Sit or lie in a comfortable position other than one you would sleep in. It is important that this position be one that is good for you. (ie, don't sit hunched over in a chair -- sit up straight, but not rigid.) The lotus position is commonly associated with meditation, but it tends to make many people too rigid, and most of us aren't flexible to do that thing with our legs. I meditate lying down on my back, with a pillow under my knees.
Now focus on something. Something that is, for lack of a better term, boring. You can't meditate while watching TV. The thing you are focusing on must have less input than you normally find stimulating. You are trying to quiet your mind, remember. Music can be good. (I like drum and bass) One of the best techniques is to focus on your breathing. Count each breath. Don't force or control your breath, just allow yourself to be calm, and observe as your breathing becomes deeper and more rhythmic. Do this until there is nothing else in your mind except the thing you are focusing on. This may take a while.
Do this for half an hour per day, and after a while, all kinds of stuff will start surfacing. Memories, fantasies, daydreams. You may even have visions. This is normal. (This phase is often called being "at the movies") Eventually, you will learn to gently bring your mind back into focus. Don't block the memories -- just let them go. Visualize "exhaling" them if you want. There's a lot of stuff that you need to let go of, and it will take time to vent all the garbage.
Eventually, you'll have a lot of clarity in your meditation. This is when you can move to the second stage. Stop focusing, stop thinking, stop stopping. Just....
...
If you're ready, you'll know what I mean. :)