Communications
Parallel tasks normally have to exchange data. There are various manners in which this can be achieved like over a network or through a shared memory bus. The real event of data exchange is normally referred to as communication regardless of the manner used.
Synchronization
The procedure of coordination of parallel tasks in actual time, very frequently associated with communications is known as synchronisation. Frequently executed by establishing a synchronization stage within an application where some task cannot carry on further till any other task(s) reaches the similar or we can say logically equivalent stage. Synchronization generally includes waiting by minimum one task, and can consequently cause a parallel application's execution time to increase.