A telecnverter increases magnification by placing another lens between the lens and the sensor increasing the focal length of your lens, typically either 1.4X or 2X depending on the converter. Extension tubes are hollow tubes that go between the lens and the sensor. By moving the lens away from the sensor, it allows thew lens to focus closer than it's minimum focus distance, since your closer, things get bigger. One thing about extension tubes though...you lose infinity, so your area of focus is very narrow.
So in short a teleconverter increases your focal length (at the cost of some sharpness, loss of contrast and 1 or 2 stops of light and depending on the lens you put it on....autofocus) and extension tubes allow the lens to focus closer (at the cost of some light).