| Often appreviated SLR.
A kind of camera construction that allows the user to view an image directly through the lens that exposes the film.
This is made possible by a complex viewing system that is built into the camera itself: light travels through the lens and is reflected by a mirror into a prism that corrects for the inverted image (a lens normally "sees" the world upside down and backwards). In order to take a picture, the mirror has to swing out of the way at the same moment the shutter opens to allow light to travel to the film. This is the reason that the photographer using a SLR camera experiences a moment of blindness upon depression of the shutter release.
The primary advantage of using a SLR camera is that what you see is (for the most part) what you get: you don't have to learn how to compose a photograph backwards as you do with a Twin Lens Reflex or upside down and backwards as you would with a View Camera. Further the problems of paralax that one experiences with a Rangefinder or Twin Lens Reflex cannot occur.
SLR cameras usually have the greatest range of lenses available to the photographer. The vast majority of SLRs are producted for the 35mm format but medium format varieties are also available and tend to be popular for their flexibility. Additionally, many medium format SLR cameras are designed with interchangeable film backs so the photographer can carry many rolls of film pre-loaded, and rather than reloading the camera at the end of each roll, they simply need to change the back - a process that takes about a second.
The real reasons one might not want to use a SLR is that they tend to be substantially larger, heavier and louder than cameras utilizing a different viewing method. In addition to the space normally needed inside the camera body to house the lens, shutter and film, space is necessary to account for both the prism and the movement of the mirror. For the 35mm varieties this is rarely a problem as even with the increased size and weight the camera almost always still fits in your hand. Most medium format SLR cameras (especially ones that allow for interchangeable backs) are substantially larger than their design competitors. Many are so cumbersome they are nearly impossible to use without a tripod. | 
