The term voice refers to the form of a verb indicating whether the subject performs an action ( active voice) or receives the action ( passive voice).
Marcy smashed the ball over the net. (active voice)
The ball was smashed over the net by Marcy. (passive voice)
Use the active voice whenever you can because it conveys more energy than the passive voice and results in more concise writing.
Use the passive voice when you don't know who is taking the action (actor), when you don't want to name the actor, or when you want to emphasize the person or thing acted upon rather than the one taking the action. The passive voice is often appropriate in scientific writing.
When we returned, the car had been towed.
I regret that a mistake was made.
Gold was discovered in California.
His mother was rushed to the hospital.
A change in structure was found in the experimental group.