What is a DSLR?

A DSLR is a digital camera, but not just any ol’ digicam. A DSLR is a
serious photo-taking tool that differs from point & shoot digital
cameras in a variety of ways. If digital photography has begun to act
as a hobby, if you’ve been hanging around Flickr,
or if you’ve just spoken to a few photographers, you’ve probably heard
a bit of praise for DSLR cameras. So just what is a DSLR, and what kind
of features does it offer?


DSLR is all About the Viewfinder


Fig. 1 - The viewfinder and lens of a P&S camera as two separate elements.
Part
of photography is composition — the placement of your subject in the
photo. You want to see what the camera sees in order to get your
composition just right, and that can’t be done by looking through the
viewfinder of most point & shoot digital cameras. Take a look at
figure 1, and observe how the viewfinder and the camera lens are two
completely different elements. The viewfinder peeks out just above the
camera’s lens, and so it’s close, but not quite the same image that the
camera sees.
Most people I know don’t even use the viewfinder on a P&S
digital camera. Instead, they compose their shots using the LCD. The
LCD is an accurate representation of what the camera sees through the
lens, but the image displayed is a very low-quality, compressed
picture. The poor quality of the picture on the LCD, combined with the
fact that you’re probably holding the camera way out in front of you,
makes it difficult to get a steady, well-composed shot.
SLR stands for Single Lens Reflex, and the D is for digital. DSLR
cameras use a system of mirrors to reflect light through the camera
lens, up to the viewfinder, and then onto the camera’s photographic
sensor. When you look through the viewfinder of a DSLR, you and your
camera are both seeing the same thing. Through the viewfinder of a
DSLR, you can accurately focus and compose the shot as you’d like it.
There is one technicality here, which I’ll mention briefly.
Inexpensive, consumer-level DSLR cameras have a crop factor. The sensor
in these cameras is smaller than 35mm film, and so the field of view
seen in the end result does not exactly match what you would
traditionally expect from the focal length you used. Instead, you must
multiply the focal length by the camera’s field of view crop factor.
For example, a Canon EOS 400D has a crop factor of 1.6, so a photo
taken through a 100mm lens will actually correspond to 160mm (100mm x
1.6). Many professional models are referred to as “full frame” cameras,
and do not have a crop factor. Whether or not full frame or cropped
sensors are better suited for digital photography is currently under
debate.
DSLR is all about Manual Controls


When you take the plunge into the DSLR world, you should be ready to
step away from the comfort of automatic and preset camera modes. Point
& Shoot cameras are great for casual photography because they’re
pre-programmed to handle common situations such as portraits,
landscapes and action shots.
While automatic and preset modes are convenient, you’ll find
yourself desiring more control over your photography as you become more
serious about the craft. Much like a car with a manual transmission, a
DSLR offers the photographer manual control over individual camera
settings, such as aperture, shutter speed, light sensitivity, white
balance, and more. A photographer possessing an understanding of these
tools can take the same picture a dozen different ways. By using these
tools to your advantage, you can add depth, feeling and emotion to your
photographs.
Photography is all about the Lens


Using the right lens to capture a scene affects your photograph in
the same manner that your choice of camera settings does. Point &
Shoot cameras have one lens that’s intended to serve its purpose in as
many different environments as possible — it’s a jack of all trades,
but a master of none. The lens on a DSLR can be detached, and the
camera body sports a lens mount that can hold onto many different types
of lenses. A DSLR allows you to choose the right lens for the right
situation — there are portrait lenses, zoom lenses, telephoto lenses,
wide angle lenses, and more.
Should You Buy a DSLR?


If you’re serious about photography, have an understanding of the
craft and an eye for light, and have been contemplating this sort of
upgrade for a while, then yes, I think a DSLR would be a good idea. You
should understand that an expensive camera will not make you a better
photographer, but a good photographer will find it much easier to
express his creativity through a quality instrument.

Source : ofzenandcomputing

This is a simple cross-section view of a SLR system


Cross-section view of SLR system.
1 - 4-element lens
2 - Reflex mirror
3 - Focal-plane shutter
4 - Sensor
5 - Matte focusing screen
6 - Condenser lens
7 - Pentaprism
8 - Eyepiece

Source : http://en.wikipedia.org/wiki/DSLR