SLR stands for Single Lens Reflex. Basically what that means is the camera views the image thru the same lens it uses to take the picture. This ensures the picture you frame will be the picture that comes out on film. It does this using a mirror and what's called a pentaprism. The light enters thru the lens and is reflected upward to the viewfinder via this mechanism. When you press the shutter release, the reflex mirror moves upward to allow the light to reach the film plane. The shutter curtain opens and closes, exposing the film, and then the mirror returns to its original position. This is why the viewfinder goes dark as you snap a picture.

Some cameras don't view the image thru the lens. You can see this most particularly in disposable cameras. If you look at the front of the camera, you'll see the lens and a small window above and generally to the right of the lens. This "window" is the front of the viewfinder. In other words, the viewfinder is basically a peephole thru the camera. The disadvantage to this is the image you see in the viewfinder is just slightly offset to the one the camera "sees". So, if you've aligned your picture in the viewfinder precisely the way you want it, you're likely to see a slight shifting of the scene in the final picture. The viewfinder on my Coolpix 880 is like this. Which is why I more often frame my pictures using the LCD monitor on the back of the camera, instead of using the viewfinder.

When we say SLR, most often we're referring to cameras that have interchangeable lenses. Although that's not entirely accurate anymore.