For starters, an SLR (single lens reflex) is a camera that lets you see directly through the lens, so you see exactly what the camera is seeing. Seeing through the lens allows a serious photographer to make fine adjustments to his/her settings and composition to get the exact photograph they are after.

SLR cameras are also capable of accepting a wide range of lenses and other accessories in order to adapt to any photographic situation. With some manufacturers offering as many as fifty different lenses for their SLR cameras, your options are only limited by your wallet.

Megapixels/Sensors

The larger camera body of an SLR allows for larger digital imaging sensors. These larger sensors produce images of higher quality than compact cameras and lead to higher megapixel counts in high-end SLR cameras.

Fast Reactions

SLR cameras are typically very quick thinkers. Modern autofocus technology allows a good SLR to track even the fastest action, exposing frames as fast as a mind-bending ten frames per second. And most importantly, with most SLR's  the photograph is exposed the exact instant you push the shutter button.

Lenses

The larger lenses of SLR cameras give photographs a more refined look capable of extremely shallow depth of field when needed. The wide range of lenses available for SLR cameras is truly astonishing, from fish-eyes on the wide end to super telephotos. Specialized lenses are also widely available for close focus work (macro lenses) and architectural work (tilt-shift lenses).

Lighting Options

SLR's come built for a variety of lighting options, from the flash you attach to the top of your camera to studio lights and remote wireless lighting.

The Levels Of SLR

SLR's come in a variety of sizes and feature sets. Most of today's digital SLR's can be classified into four categories. The model numbers, amount of megapixels, and other features change every several months, but these categories seem constant to the digital SLR camera market. Listed in order of typical cost (low to high):

1. Entry level amateur
2. Mid level amateur
3. Professional - high speed
4. Professional - high quality

SLR - Pros

The SLR remains the dominant camera of the professional, and for good reasons: lens quality, larger image sensors, faster reaction times and autofocus, adaptability. A good SLR system provides you with the ability to tackle any situation you might be faced with. 

SLR - Cons

You'll always pay more for an SLR. And SLR's are larger and heavier than compact cameras. Their professional look often attracts unwanted attention. A disadvantage to some, the LCD on your typical SLR camera can't be used as a live preview when composing your photographs.