The voice of a verb refers to the form of the verb used in relation to what the subject is doing. In English there are only two voices-- passive and active.
The passive voice of a verb simply means the form of the verb used when the subject is being acted upon rather than doing something.
The passive voice is formed by taking the appropriate tense of the verb to be and adding the past participle.
Active Voice: The committee reviewed the project.
Passive Voice: The project was reviewed by the committee.
In most writing, use the active voice. It is more direct and less ambiguous.
The passive should be used only if the doer is unknown or unimportant, or if more emphasis is put on the receiver of the action than the doer.
Doer unknown: The Jones' car was stolen last week.
Doer unimportant: The bells were rung to announce the wedding.
Emphasize receiver: Tony was hit by a fastball.