From Wikipedia
http://en.wikipedia.org/wiki/Canon_Extender_EF
From Wikipedia
http://en.wikipedia.org/wiki/Canon_Extender_EF
Just know that not all between the lens and camera lens converter will work with EFS lenses. Most aftermarket converters are listed for EF lenses only.
Lets be honest. Modern cameras can resolve so much detail that you get a better image without using a converter than using a cheap converter on a cheap lens. In the days of film, and especially "slides" where the picture taken was the image, tele converters did a useful job. Now, as has been said above, only where the lens and converter match can good image quality be expected.