About F-Stop
When I was trying to explore more about DSLR I came across the F-STOP from one of the blog . Some one wrote the comment about F-STOP like this
What is meant by f-stop?
A. The focal length of the lens divided by the diameter of the aperture (as seen from the front). It is also called an f-number, and is written like f/8, which means the aperture diameter is 1/8th the focal length.
The term is used both in regard to the maximum aperture of a lens and in regard to the aperture selected in a specific situation. The brightness of the image on the film is inversely proportional to the f-number squared. The depth of field increases but diffraction is worsened when using a large f-number. The effective f-number for all 3 effects changes if the lens is focused extremely close. The term "stops" purportedly comes from old technology in which the aperture was selected by turning a wheel with various sized holes in it, each one of which let in twice the light of the preceding one. Thus the phrase "open up a N stops" means to change to an aperture allowing in 2^N times as much light, and conversely with "stop down N stops".
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Sounds a little more plausible than "fenestra", doesn't it?
Then other guy explained in more interesting way
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After searching on this for hours, I have to go with it being a math equation for Focal Length ("F") over ("/") diameter. A lens with 14 inch "focus" (focal length) which is four inches in diameter he is talking about an f /3.5 lens -- (14 / 4) = 3.5 It's origins appear to go back to the mid 1800's and was used in a letter from Irving Pobboravsky to John William Draper. In it, he gave the above formula with an accompanying detailed explanation of what the F/ on the new lens stood for ("focal length over diamter").
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This is what I read in a magazine recently. The f stands for fraction of the focus length of the lens. In the old days when a 50mm lens really was 50mm physically, an f/2 is an aperture 1/2 the size of the focal length (not sure its diameter or what).So for the same light gathering power, ie same f stop: f/2 of a 50mm lens, the aperture has to be 25mm large f/2 of a 200mm lens, the aperture has to be 100mm large Of course modern optics meant focal lengths no longer need to have a direct relation to the physical size. The f number is now only a indication of light gathering power. However, a longer lens still requires a larger aperture than a shorter lens, which is why it fast telephoto lenses costs so much
which was a basic model from CANON in DSLR series and finally we bought that. I am not sure for past one year I am using and still trying to learn. Downloaded couple of CBT J (Yeah they have CBT for this) to learn it and I even knew that this is not something which you can learn from CBT. But to know the DSLR you need to understand terminology like ISO,Noise, Exposure, Zoom, Overexposed etc. Then I came to know college of mine knew photography well and was using same DSLR as mine. One more distance college was an amateur photographer who was using EOS and was using 
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