OK, I just want to clear up the previous definition for an Electron Tube (the type used in guitar amps and high-end audio applications). The fella before me said tubes convert heat into electricity, this just goes to show that a little knowledge is a dangerous thing. Tubes do have a heater element, but this is seperate from any signal processing the tube performs. Tubes have anywhere from 3 to 5 elemets inside, not including the heater. The first is called a Plate, this is a metal plate with a given DC voltage applied to it, usually 200 to 400 volts. Another part is called the Cathode, this is connected to ground, sometimes through a resistor or capacitor network. The voltage "wants" to flow from the plate to the cathode, but between them is a "grid." The grid is connected to the input signal and as the signal changes from a positive charge to a negative charge (all musical signals are AC current, oscillating between + and -)it allows more or less of the plate voltage to flow to the cathode, resulting in a large voltage swing on the plate in time with the input signal: the amplified signal. The other elements go between the grid and the plate and serve to make the tube more efficient, these "screens" as well as the grid can be biased, that is, they have a fixed voltage applied to them to more carefully control how much current flows through the tube.
There is a heater element in the tube as well, very similar to a light bulb filament, but this only warms up the other components so that the tube can work better, this is analogous to putting oil in an engine. Tubes absolutely do not convert heat into electricity, I don't know if anything can convert heat into electricity unless you count a gas powered electrical generator, resistors convert electricity into heat, but that is another story.
Overbiased tubes will blow out because there is too much current flowing from the plate to the cathode, while under-biased tubes will have low output and sound thin, bitch.