Telephoto is commonly referred to telephoto lenses (or tele), for SLRs and some point and shoot cameras. Telephoto lenses on SLRs could range from the mild 100mm to strong 600mm, or even 800-1600mm, with a teleconverter. Tele lenses tend to be very large, depending on how fast they are and thier focal length. Canon's L-series "white lenses" seen at sporting events are commonly f/2.8 for 400mm, or f/4 for 600mm (there are tons of focal lengths and apertures in between). Common optical problems that plague most teles are vignetting, barrel distortion, Pincushion distortion, and chromatic aberration (also known as purple fringing). Most telephoto lenses, and all fast ones, are very expensive. There are cheap ones out there, like the Nikkor 70-300 f/4-5.6G (which you could easily get for about $170). But if you want a pro quality paparazzi/sports style lens like Canon's 600mm f/4L IS USM, you'll have to invest a little under $9000.
Although expensive, telephoto lenses are really fun to shoot! Sell a kidney!