What is triode, pentode or tetrode?

A triode is the description given to the type of tube utilised and refers to the amount of elements it has, which for a triode it has three, a Cathode, Anode and Grid. A tetrode has four elements, a Cathode, Anode, Grid and Screen Grid. A pentode has five elements, a Cathode, Anode, Grid, Screen Grid and Suppressor Grid. TT prefers pentodes in output stages as these extra grids stop the problem of secondary emission of electrons. This happens when electrons that have been emitted from the cathode bounce back and get re-emitted a problem with triode power tubes.