ACTIVE AND PASSIVE VOICE

Active voice

In most English sentences with an action verb, the subject performs the action denoted by the verb.

    These examples show that the subject is  doing the verb's action.

            The man must have eaten five hamburgers

                        The man (subject) is doing the eating (verb)

            Marilyn mailed the letter

                        Marilyn (subject) is doing the mailing (verb)

            Colorful parrots live in the rainforests

                        Parrots (subject) are doing the living (verb)

Because the subject does or "acts upon" the verb in such sentences, the sentences are said to be in the active voice.

Passive voice

One can change the normal word order of many active sentences (those with a direct object) so that the subject is no longer active, but is, instead, being acted upon by the verb - or passive.

    Note in these examples how the subject-verb relationship has changed.
            Five hamburgers must hane been eaten by the man
                        Hamburgers (subject) are being eaten (verb)
            The letter was mailed by Marilyn
                        The letter (subject) was being mailed (verb)

Because the subject is being "acted upon" (or is passive), such sentences are said to be in the passive voice.

Sumber : http://www.towson.edu/ows/activepass.htm