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Active voice is the grammatical construction used where a subject does the action designated by the verb. In other words, this is the most common, straight-forward way of structuring a sentence in English. For example: The cat ate the mouse (The cat = subject, ate = verb, the mouse = object. The cat did the eating, therefore this is active voice).
The passive voice is the grammatical construction used where the object of the active sentence (the mouse) appears as the subject. For example: The mouse was eaten by the cat. This is usually used to emphasise the object. Someone may ask "What was eaten by the cat?" and you could then emphasise that it was the mouse by saying "The mouse was eaten by the cat".
Take a look at this link, it should explain all you need to know about passive voice: http://en.wikipedia.org/wiki/English_passive_voice
