"The manager wrote the report yesterday." The person acting is the manager.
In passive voice, the writer does not specify who is acting: "The report was written yesterday." It could have been written by the secretary, George Bush, or the manager--we don't know.
The sentence is still in passive voice if the writer specifies the actor later in the sentence: "The report was written yesterday by the manager."
Use passive voice sparingly
Passive voice is still appropriate for some sentences. However, business writers should use the passive voice very sparingly. It may make the writing unclear by keeping the identity of the actor secret.
Passive voice is also a poor choice for sentences because it often sounds awkward and evasive. Readers may interpret passive voice as an attempt to avoid admitting responsibility, as in the following example:
"A mistake was made that resulted in an overcharge to your account that has now been corrected and will be shown on your next statement."
Active voice is more direct and concise than passive voice. Active voice sounds more responsible: "Our data entry clerk made a mistake and overcharged your account, but she corrected the entry. Your next statement will show the correction."
Usually use passive voice when you do not know the actor, you want to hide the identity of the actor, or the actor is not important to the meaning of the sentence.