Use the passive voice to:
Call attention to receiver of the action rather than the performer:
The professor was hit by three snowballs.
Point out the receiver of the action when performer is unknown or unimportant:
A love letter was slipped under the door.
The signs will be posted.
Avoid calling attention to the performer of the action (known as the "institutional passive"):
The fines will be collected on Monday.
In active voice, on the other hand, the subject performs the action of the verb:
The cats climbed the curtains.
Grandma and Grandpa raced grocery carts across the parking lot.
Active and passive voice in writing:
The choice between using the active or passive voice in writing is a matter of style, not correctness. However, most handbooks recommend using active voice, which they describe as more natural, direct, lively, and succinct.