A tele converter is designed to increase the focal length of the lens. It contains glass elements and is available in magnifications upto 3x.The most popular being 1.4x or 2x. By using a 2x converter one can double the focal length of the lens. A 50mm lens will work as 100mm lens.The introduction of the tele converter between the lens and the camera body almost always lead to optical quality loss.

Extension tubes are hollow tubes that fit between the camera body and the lens. They are often available in sets, which can be used singly or together. The longer the tube, the higher the magnification. However, the longer the tube, the greater the distance light has to travel. This needs a compensation in exposure
 

    
   
As the name sugests, this is only a tube, which helps in taking the lens closer to the subject, enabling close focuing and hence more magnification. It reduces the minimum focusing distance.
Teleconvertor increases the focal length of the lens as it contains lens elements.
Magnifies a close up subject.
Makes the angle of view narrower.