DSLR stands for Digital Single Lens Reflex or a Digital SLR camera that uses a mechanical mirror system and pentaprism to direct light from the lens to an optical viewfinder on the back of the camera. The basic operation of a DSLR is as follows: for viewing purposes, the mirror reflects the light coming through the attached lens upwards at a 90 degree angle. It is then reflected three times by the roof pentaprism, rectifying it for the photographer's eye. During exposure, the mirror assembly swings upward, the aperture narrows and a shutter opens, allowing the lens to project light onto the image sensor. A second shutter then covers the sensor, ending the exposure, and the mirror lowers while the shutter resets. The period that the mirror is flipped up is referred to as "viewfinder blackout". A fast-acting mirror and shutter is preferred so as to not delay an action photo. Source:Wikipedia
Since DSLR allows the use of interchangeable lenses, it is rather bulky and heavy and requires extra caution in carriage and handling. Lately as an enthusiast, I find the need to carry with me on a daily basis a pocketable camera which I can take out easily when shooting opportunities present itself. Thus, I found myself owning a Canon PowerShot S90. This little point and shoot camera was dubbed by Ken Rockwell as The World's Best Pocket Camera.
I like Nikons and I'm a staunch critic of Canons when it comes to DSLRs but when speaking of compact cameras, Nikon models suck and Canon has the better line up. Sometimes, sticking it out with your favorite brand would limit your photographic options and knowledge on what the other brands have to offer. These are the reasons why I tried other brand/model like the Ricoh GRD3 and this current gem among the smaller-sized sensor compacts, the Canon S90. Like the GRD3, the S90 sports a similar 1/1.7 inch high-sensitivity CCD sensor and designed to compete with the popular Panasonic Lumix LX3.
Considering the smaller size sensor inside compact cameras compared to APS-C or full frame cameras, image quality will of course suffer aside from the obvious lack of other features found only on more expensive DSLR models. But if you only post your pictures on the net and printing large copies of the images you took is not one of your preferences, then compact cameras should do the trick.