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A lens that's also called standard because it has a focal length roughly the same as the diagonal of the film format and is around the same magnification as the naked eye.
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This is technically known as a catadioptric lens and has an unusual construction of mirrors and lens elements. As well as glass elements there are mirrors at the front and rear to fold the light as it enters the lens. Although this results in a body that's a little wider than normal, advantages are: 1. The lens is usually only half the physical length a regular lens of the same focal length would be, and 2. It's much lighter. Disadvantages are: 1. There is no adjustable aperture, so the user is forced to take all of his photos at a permanent aperture setting, usually f8, but sometimes even f11, which means you need plenty of light for taking photos; 2. Practically all mirror lenses use manual focus; 3. Highlights that are out of focus are, in some situations, shown as doughnut shapes (although some actually like this characteristic and consider it an advantage, not a disadvantage).
Another name for a shift lens that has a sliding front panel so the lens can be raised or lowered from its normal position to correct for verticals when shooting from high or low angles. Some also have a swing facility to control depth of field using the Scheimpflug rule.
Cameras offer different levels of image quality which is determined by lens quality and the resolution (number of pixels) delivered by the CCD. Basic cameras have VGA resolution CCDs that record images with 640x480 pixels. Next up are SVGA models that record 800x600 pixels and then the XVGA models that create pictures with over 1024x768 pixels. The latest super megapixel models go way beyond these figures – currently creating images up to 3040x2016. If you're buying the camera to take pictures for use on screen you only need to buy a VGA model, but if you demand photo quality you need at least a megapixel variety and even then the quality is only good to about 5x7inch from a normal inkjet printer.