I'm not sure if threads like this has been started before. Why not we list down the common photographic terminologies / glossary here. Very usefull, especially for newbies. I'm starting with a few and hope you guys will expand it for the benefit of CSers.

Aperture : The adjustable opening in a camera lens that -like an eye's iris-
controls the amount of light that reaches the film. The size of
such aperture is called the f-stop, like f/2.8. Source : Nikonians.com

Pixel : A pixel is one of the many tiny dots that make up the representation
of a picture in a computer's memory. check here Pixel

Focal Length: The focal length of a lens is defined as the distance in mm
from the optical center of the lens to the focal point, which is
located on the sensor or film if the image is "in focus". Dpreview