Yagi antenna is a dipole antenna with parasitics to control the beamwidth. there are two different types of parasitics 1. reflector 2.directors.
the main element is a dipole whose length is approximately a half wavelength(0.45~.47 lambda), reflector is the element behind the main element,whose length is bit longer than main element and the directors are placed infront of the main element and in the intended direction of the beam.
find this small explanation below.should you need any further information,please feel free to contact