Digital Camera - SLR vs Compact

SLR is an acronym for a Single Lens Reflex. The difference between an SLR and a compact camera is that the SLR will allow you to use a variety of lenses with different focal lengths.

Differences Between SLR and Compact Cameras (Basic)

SLR	Compact
Interchangeable lenses	One attached lens
TTL (Through The Lens) viewfinder — when you look through the viewfinder of a SLR, you are seeing what the lens sees. If the image is out of focus, the photo will be out of focus.	The viewfinder is separate from the lens and shows you an image of the scene in front of you. The lens can be out of focus, but your view in the viewfinder will be sharp.
Bulky and heavy	Sleek and small, except the super-zoom camera
The camera of choice for professional photographers	The camera of choice for the average consumer
All can use an external flash unit	Very few can use an external flash
All offer complete manual control	Not all offer manual controls
Capable to capture pictures in RAW or JPEG format , some can capture both in one shot	Mostly capture pictures in JPEG format only, seldom support RAW format

DSLR cameras' sensor are much bigger and consequently much more expensive than the thumbnail-size sensors found in campat cameras. Larger sensors are the secret to why 6 megapixels from a DSLR camera beat 6 megapixels from a compact camera. To spread the same number of pixels over a larger sensor area, the pixels must be bigger. These bigger photosites gather more light, so they produce less-noisy images, capture greater dynamic range, and perform much better at high ISO settings.