Now, this is FAR easier if you understand how Transistors work, and how CRTs work.
Valves were used in the first amplifiers......

Valves are THERMIONIC devices; evacuated "Tubes" with a Cathode and Anode at each end. 

A potential is applied between the two, AND the Cathode is Heated, making it release electrons, which are attracted to the Anode and travel there ('coz it's a vacuum).
Next step; There is a metal "mesh" placed in between the Anode and Cathode, called the 'grid'.
If un-charged, the grid has no effect on the current flowing between Anode and Cathode, HOWEVER, even a small charge on the Grid can vary the flow of the 'main' Current.

SO, it should now be apparent how valves can be used for basic amplification!

Different types of Valve have different numbers of grids inside and are known by the TOTAL number of electrodes within the tube; single-grid valves are called "Triodes" while three-grid valves are "Pentodes".

Also interesting is that TRANSISTORS have "collectors" and "emitters", so-called because of the actions of the components in the valves!!!