Check out CNET’s Digital SLR Buying Guide. Here’s an excerpt:

So what is a digital SLR anyway?

The term digital SLR is short for digital single lens reflex, so named because these types of cameras use a mirror positioned behind the camera lens to direct light toward the viewfinder when you’re composing a photo. When you release the shutter, the mirror swings quickly out of the way, letting light from the lens travel straight to the sensor and momentarily blacking out the viewfinder. The viewfinder in an SLR incorporates a prism–usually a pentaprism–that flips the incoming image around so that you can see it right side up and bounces it onto the focusing screen where you see it.

The SLR design allows one camera to accommodate a very wide range of lens focal lengths, and that’s the biggest reason that SLRs dominate serious photography. The explanation? With a non-SLR camera, you have to match the angle of view of the “taking” lens with that of the “viewing” lens. That’s easy with a fixed lens or a short-range zoom, but it requires increasingly complex and expensive viewfinder mechanisms as you try to cover a wider range of focal lengths. With an SLR, you avoid this problem because the taking and viewing lens are one and the same.

Most dSLR models beyond entry-level models incorporate a Live View mode, which allows the photographer to use the LCD to compose shots the same way they can with a snapshot camera. The most basic implementations generally lock up the mirror, with the prism diverting the image to a small sensor that feeds through to the LCD rather than to the capture sensor. This does tend to hurt performance, however. Early versions required that you focus manually when in Live View mode, but current models use contrast autofocus.

Click here to read the full article, which includes types of SLR cameras, advantages/disadvantages of SLR, and a guide to available features.