Active voice: The batter hit the ball.
Passive voice: The ball was hit.
Notice that in the active voice the subject performs the action.
In the passive voice, the action is performed on the subject.
This is the real difference between the two.
You can only use the passive voice with the verb is transitive. Try to make a passive out of a man sitting down. He can sit "on a chair" but now we have introduced a prepositional phrase. The paper was written. The song was sung. This are transitive verbs used in the passive voice.