A lens primer article. Pretty informative for beginners unfamiliar with exchangeable lenses.
Though a bit imprecise that he consistently uses the term "entry-level" for a reduced-frame (APS-C) camera (Nikon D90, Canon 50D etc), and "pro-level" for a full frame camera (Canon 5D, Nikon D700 etc). True, the size of the sensor is roughly related to the level and price, but these are not official terms, and for example the Nikon D300 is definitely a pro-level camera even though it has a reduced frame sensor.
Here's another good basic article on what lenses are.