What follows is excerpted from the highly recommended 'Grammar Girl' site.
Don't hesitate to add your own comments.
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What is active voice?
In an active sentence, the subject is doing the action. A straightforward example is the sentence "Steve loves Amy." Steve is the subject, and he is doing the action: he loves Amy, the object of the sentence.
What Is Passive Voice?
In passive voice, the target of the action gets promoted to the subject position. Instead of saying, "Steve loves Amy," I would say, "Amy is loved by Steve." The subject of the sentence becomes Amy, but she isn't doing anything. Rather, she is just the recipient of Steve's love. The focus of the sentence has changed from Steve to Amy.
Passive voice isn't wrong, but it's often a poor way to present your thoughts.
Another important point is that passive sentences aren't incorrect; it’s just that they often aren't the best way to phrase your thoughts. Sometimes passive voice is awkward and other times it’s vague. Also, passive voice is usually wordy, so you can tighten your writing if you replace passive sentences with active sentence.