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.

Types of digital SLRs

• Interchangeable-lens full system digital SLR

Nikon D300
These are what most people mean when they say "digital SLR," and they are the primary focus of this buying guide. As the name implies, the ability to remove one lens and replace it with another--to go from, say, ultra-wide-angle to supertelephoto--is what sets these cameras apart.
Examples: Almost all of the digital SLRs available today are of this type. Here you'll find reviews of the prosumer and professional models that we like best.

• Fixed-lens digital SLR

Olympus E-20N
The lenses on these cameras can't be removed, which limits their versatility. The best known of these models use a semitransparent, nonmoving mirror to bounce some light to the viewfinder while letting most through to the sensor, which means you can use their LCDs for composing.

• SLR-like or SLR-style
  

Canon PowerShot S5 IS
These are standard digicams that use an electronic viewfinder (EVF)--just a small LCD--in place of an SLR's pentaprism or a point-and-shoot's optical finder. They can't truly be considered SLRs because they have no mirror, and we've yet to see an EVF that approaches the image quality of a decent pentaprism viewfinder. Most cameras of the type have extremely long zoom lenses and cameras are sometimes referred to as ZLRs or megazooms.
Examples: Megazoom, SLR-style cameras

Taken from http://reviews.cnet.com