A little help with extenders.

To clarify:
Extender - this is what Canon have labled their teleconverters as, but everyone else (shops and other manufacturers) call them teleconverters and most canon shoots still call them teleconverters too.
These essentially magnify your image and thus give you a longer focal length for your lens - the amount you get is the number of the teleconverter (1.4 or 2) times the focal length of the lens you are attaching it to. So adding a 2*teleconverter (extendor) to a 100mm lens would give you a 200mm lens.
However they are not ideal on all lenses and the 100-400mm is not going to work well with the 1.4 and would be very hard to use with the 2*TC. As USAyit said you idealy want to use them on prime lenses or very high quality zooms (eg a 70-200mm f2.8) and on zooms only the 1.4 will give acceptable image quality in most situations.

Extension tube - a different item here, this is a hollow tube that allows your lens to focus closer and thus achive a greater level of magnification than previously possible. However in doing so you lose the infinty focus of the lens. Depending on the lens and the amount of tubes you add you can have anything from only a few feet down to a few millimeters of distance that you can focus over.
Typically these are used for close up or macro work - and a common setup is a "cheapmans macro" which is a 50mm f1.8 lens with around 50mm worth of tubes; the rough maths is that adding the same length in tubes as thefocal length of the lens will give you 1:1 (true) macro magnification; of course in the real world you don't want to go adding more than around 50mm of tubes because you do lose light (image quality) as well as having a more difficult setup to use.
Note that Canon tubes are not worth getting since they arevastly overpriced for a tube with nothing in it and most people will use "Kenko AF tubes for Canon". The AF tubes are important as there are even cheaper tubes on the market, but they lack the AF contacts in teh tube construction. This means that not only can you no longer use auto focus, but you can also not control the aperture blades on the lens - this meaning that you have ot shoot wide open (biggest aperture smallest f number) which is not ideal for closeup work.