Teleconverter:  A secondary lens that mounts between your lens and the camera, which makes the effective focal length of your lens longer (magnifies the image), but also reduces the effective aperture (lets less light in). 

They come in two strengths. A 2X teleconverter doubles the focal length (magnifies the image 2X), but cuts down the amount of light the lens takes in by a factor of 4 (only 25% as much light reaches the camera).  The other common strength is a 1.4X teleconverter.  Why this strange number, you ask?  Well, it turns out that a 1.4X teleconverter cuts the light reaching the sensor (or film) by half, which can be compensated for by increasing the aperture by exactly one F-stop.  It is also possible to stack teleconverters, putting two of them on a single lens.  For example, a 200 mm lens with two 2X teleconverters will have an effective focal length of 800 mm, although it will only let in 1/16 as much light as the original lens.

Teleconverters are very useful because you can double the focal length of a lens without having to carry a second lens around with you. However, a teleconverter will manigfy any imperfections in your lens. So if your lens isn't that sharp to begin with, the lack of sharpness will be much more evident. Also, the lens becomes much slower. With the 2X teleconverter, you have to use a shutter speed FOUR times as long for a given light condition at a given aperture.