Shutter speed is how fast your shutter opens and closes to take a picture on your camera. Shutter speed also has a lot to do with exposure. The higher the shutter speed (1/2000 example) the less light.

You can adjust the shutter speed of Canon EOS cameras so there isn't a set shutter speed.

The shutter speed setting controls the picture exposure, or how dark or bright the picture will be. Wikipedia has some very good information on shutter speed and aperture.

It doesn't necessarily inprove your picture. The higher the shutter speed, the less light, also the blur in a photo. A higher shutter speed will freeze a photo. A lower shutter speed will blur most motion and will increase chances of blur.

Shutter speed has a lot to do with your overall exposure. A higher shutter speed will decrease the light a lot. I was in best buy playing around with a camera and turned the camera shutter speed to 1/4000. After taking the photo, the preview was black. I realized it was because of how much higher I increase my shutter speed. Shutter speed also affects motion in a photo. A higher shutter speed will freeze all action in a photo a will less likely have a blur to it if you have a shaky hand. A lower shutter speed lets in more light while the shutter is will opened so the entire time that the shutter is open, the camera takes in all that light. Increasing chances of blur, you can also use this for crazy, funky effects! If your taking a picture of a waterfall with a low shutter speed, your gonna get a blur, which can typically look pretty epic sometimes too.