Apparent power is electrical power summed up of two powers, real power and reactive power. Real power is power which is actually used by devices , machines, instruments etc in form of heat or magnetic field. it also can be used in other farms as storages like batteries, capacitors, inductors etc.
AC power has two components real power and reactive power. Apparent power is power which appears on the terminal of ac load like motors mostly. Apparent power is measured in VA,KVA MVA. All the generators are rated in KVA , VA or MVA. Apparent power is sum of reactive and real powers. Reactive power is always present but it is not used. Reactive power is always transmitted back to the system or generator while real power is used by the load. All inductive load or lagging power factor loads transmits reactive power back to the system.
Only resistive load like lights or heaters etc don’t use reactive powers but real power.
In figure you can see the apparent power, real power and reactive power. Real power is also called Active power and True power. Real power is measured in KW and denoted by S. Reactive power is measured in KVAR,VAR,MVAR and denoted by Q
Here is another picture
which describes best about electrical power triangle.