For present purposes, a tube is simply a component capable of amplifying a signal.
Although a single tube can be configured as a complete amplifier, most systems are divided into a pre-amp, and a power amp. The preamp increases the voltage of a faint signal, typically taking a 100mV peak-peak signal (instrument level) and boosting it up to about 1V peak-peak (line level). The power amp then takes that signal, and increases itscurrent, giving it enough power to move a physical transducer (speaker). The power section is usually isolated from the speaker by an output transformer.
In the simplest topology, known as Class-A, a single output tube amplifies the entire output waveform. It has a characteristic tone that some prefer, particularly for high-end audiophile gear, but suffers from heavy power consumption, and a lower output.
Class-AB, also known as ‘push-pull’, employs power tubes in pairs, one amplifying the positive swing of the output waveform, and the other amplifying the negative. Since one side is always idling half the time, this configuration is a bit easier on tubes. It has more punch than Class-A, but suffers from a tendency to create unwanted crossover distortion if the tubes aren’t closely biased. This topology requires the addition an intermediate stage known as a phase inverter. This generates two copies of the wave form, 180-degrees out of phase, one feeding each side of the power amp. Although this can be accomplished by a single tube, the most common configuration, known as the long-tailed pair), employs two triodes (usually configured in a single bottle known as a ‘dual triode.’)