"mm" means millimeters, and is a measure of the focal length of a lens. Your example of a 28-90mm lens is one that zooms from a wide angle of 28mm out to a telephoto 90mm. The term "f/4-5.6" refers to the maximum aperture of the lens. In this case, this lens can open up to f/4 at 28mm, and to f/5.6 at 90mm. The lower the f-stop number, the wider the lnes can open, and therefore the more light that can strike the film. f/4 is wider than f/5.6, and so you can use faster shutter speeds. There are many web sites that talk about camera basics. The only one I can think of off the top of my head is Silverlight Photography Tutorials, http://www.silverlight.co.uk/tutorials/toc.html Hope this helps.