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Common collector is also called an emitter follower. It has very close to unity voltage gain (.99 -> .998) and a high current gain. This allows it to be connected to a source which has a high impedance, such as a crystal gramophone pickup and a low impedance such as a long cable.
Common base connection is the opposite, it has unity current gain , low input impedance and a high output impedance and low distortion. It has a high voltage gain.
Common emitter connection is intermediate, medium input impedance (3k?) highish output impedance (10k?), large current gain and medium voltage gain.
Frank
it really isn't used for impedance matching. It is used for buffering. As mentioned, it has a high input impedance, which minimally loads the driving stage. It also has a low output impedance, making load variations less important.
this can also be useful for impedance matching in some cases, as the impedance is then set more by the passive elements. (series output resistor and baising resistors).
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