Conventional radar, in the simplest terms, consists of an
antenna and a reflector that emits high frequency electromagnetic
fields. Because of this, it cannot be steered without actually
rotating the radar itself. This means you have mechanical parts and
mechanical parts fail with time, along with the emitter array.
Phased array radar systems on the other hand, do not need
mechanical steering and come in two categories; passive and active.
Passive phased array radars simply consist of several emitters
arrayed in an array (or simply think of several rectangular blocks
together), while active phased array consists of several Galenium
Arsenide chips together. If you recall diffraction grids
(diffraction of light through a grid), the principle is similar.
Because each unit in the array emits an EM field, they will
superposition and by adjusting the intensity of the EM field from
each unit, you can steer the radar "electronically". The advantage
is higher power, resolution, accuracy. and speed.