Voice indicates whether the subject is doing the action of the verb or receiving the action of the verb. If the subject does the action, the sentence is in active voice. If the subject receives the action, the sentence is in passive voice.
- Active: Chris ate all the cookies.
- Passive: All the cookies were eaten by Chris.
- Active: The snow covers the cars.
- Passive: The cars are covered by the snow.
Notice that in each case, the subject and the direct object trade places. Passive sentences that name the doer of the action typically use the preposition by when they name the doer.