Shutter speed...

Shutter speed controls how long light is allowed to enter the camera (and hit the sensor) to form an image. The term refers to a moving part inside the camera called a shutter. Think of this as a little door or gate that keeps light entering via the aperture from hitting the sensor. When you press the shutter button (the button you press to take pictures), this little door opens (very, very quickly usually) and an image is captured on the sensor. For more information than you're likely to ever need on shutters, click here.

I usually think about shutter speed in two situations:
1. At night to take long exposures (an advanced topic I'll cover later)
2. To manipulate how much blur or crispness I capture in a picture that contains moving stuff, e.g., people, water

That having been said, 95% of the time I don't think about shutter speed at all. I use either "full auto" or "Av" modes so the camera chooses the shutter speed. In my experience, it is much better at this than I.

Anyway, most of the time you'll want the shutter to open for a fraction of a second. You can play with shutter speed a little to see the effects you get. I think the easiest way to do this is to use full manual mode. If you're new to photography, I would recommend using full manual mode to play around only. As its name implies, in this mode you control everything. I've taken easily over 100,000 shots and can still screw this up! More on full manual in a future post.

By the way, when I first started understanding the key variables that effect exposure, I used full manual ("M") mode a lot. I thought it made me some kind of hardcore photographer. All it really made me was a stress case with a lot of bad pictures. I now use full manual on very, very rare occasions as the Rebel XT is really good at choosing the right shutter speed (see above). I would assume most amateur photographers are in the same boat.