A triode(from three electrodes) was the vacuum tube equivalent to a transistor. It consist of a vacuum tube with an anode, a control grid and a cathode filament.
simplified buildup

    O-----.  : |
           ) : |
          (  : |
  Glow     ) : |
  current (  : |
           ) : |
          (  : |
           ) : |
    O-----'  : |
         |   | |
         O   O O

         C   G A


C  Cathode
G  gate/grid
A  anode

To the cathode filament a glow current is applied, this is because electrons is more 'willing' to leave a heated electrode. By applying a voltage to the grid, a flow of electrons to the anode is created. And by changing the grid voltage, the flow can be controlled.

The triode is still used in some amplifiers, but due to lower efficiancy it has mostly been replaced by transistors.