By Electrodynamics we understand the study of time-varying electric and magnetic phenomena.
These phenomena are caused by static electric charges whose amplitudes varies over time or by moving electric charges. The interaction forces between these charges are due to the electromagnetic field (which is composed by the electric and magnetic fields reunited into a single one). In electrodynamics there are deducted laws describing this electromagnetic field starting from the study of electric and magnetic simple phenomena. By these laws electrodynamics predicts the existence of the electromagnetic waves (see equations of Maxwell) which was later proven experimentally.
For this, first electrodynamics study the laws of electric field and magnetic field generated by static and moving electric charges (electric currents) and then generalizes these laws by postulating (or in other words stating or imposing) the existence of electric and magnetic vector potentials, and finally deduce the equations of electromagnetic field (Maxwell’s laws) and postulate the conservation laws for its the energy and momentum.
Thus Electrodynamics unify all electrical, magnetic and optical phenomena as being actually an expression of a single electromagnetic field. Due to this unification electrodynamics is the theory from which the theory of relativity was later developed .