This document defines a linear accelerator and describes its components and generations. It begins by defining a linear accelerator as a machine that uses electromagnetic waves to accelerate charged particles like electrons to high energies. It then describes the three generations of linear accelerators from early bulky models to current compact highly reliable designs with improved treatment capabilities. The document concludes by describing the major components of a linear accelerator including the modulator cabinet, console, drive stand, klystron, waveguide and others.