All tubes follow the same basic principles. Inside a tube’s glass enclosure, you have 1) a vacuum and 2) at least two electrodes (but often more, depending on what type of tube we’re talking about). Each electrode has a specific job: either releasing electrons, attracting electrons, or slowing down or speeding up the flow of electrons. We’ll talk about these jobs later. For now, the main thing that tubes are intended to direct electrical current from A to B in some way that is useful to the circuit.

There was a time when vacuum tubes were a huge industry. Just about every time of consumer electronics required tubes, and manufacturers competed for a slice of the pie by offering greater resiliency, lower noise, longer lifespans, etc. And then cheap silicon transistors were invented.