It is the equivalent of the transistor in your solid-state device. It takes a music signal and amplifies it. It is what makes the amplifier amplify. For the how-do-the-work part, yes, do check out the articles posted above.
It above referanced link is an older one but still just about the best introduction. It is a bit funny, the writer spends so much time talking about how out-dated tubes are and then, at the very end, he retreats...just a bit.
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