The main difference is :

In active voice the subject acting while in passive voice the subject is the target of the action.

Passive: The orange was peeled by Sandra.  (The orange didn't peeled itself, it was Sandra who peeled it.)

An passive voice sentence is a sentence that muse have verb "be + p.p."

e.g. Twelfth Night was written by William Shakespeare.

An active voice sentence is any sentence which doesn't follow the pattern above.

e.g Joey writes a letter to his friend.