Vacuum tubes are electrical devices that fulfil many needs in circuits. They could have been diodes, switches, rectifiers, or, most commonly, amplifiers.

If you are familiar with transistors, vacuum tubes behave like field-effect transistors.

Major Types of Tubes
Diode (2 electrodes)
Triode (3 electrodes)
Tetrode (4 electrodes)
Pentode (5 electrodes)

What is bias?
Tube bias means regulating the current that is needed to operate. Too much or too little, and the tube may burn out.

There are so many different tubes. What do the tube names mean?
There are several different naming schemes for vacuum tubes. The most common is formatted like this:
6AX7
6L6
35W4
The first number is the filament voltage.
The letters in the middle are an arbitrary model number.
The final number is the number of elements