In the active voice, the doer of the action is the subject of the sentence.
Bill threw the ball.
In the passive voice, the object, or receiver, of the action becomes the subject. Often we don't even know who the doer is.
The ball was thrown.
The ball was thrown by Bill.
The active voice adds liveliness and conciseness to writing. The passive voice adds wordiness and clouds meaning.
A gun was fired by the police officer, and the gunman was wounded. The bank robbery was foiled, and the hostages were freed.
The police officer fired his gun, wounded the gunman, foiled the bank robbery, and freed the hostages.
Generally write in the active voice, avoiding the passive voice as much as possible. Save the passive voice for when you really don't know who performed an action or the rare case in which it is necessary not to identify the doer.