Active Voice
In sentences written in active voice, the subject performs the action expressed in the verb; the subject acts.

In each example above, the subject of the sentence performs the action expressed in the verb.
Would you like to see examples of all the verb tenses in active voice?
Passive Voice
In sentences written in passive voice, the subject receives the action expressed in the verb; the subject is acted upon. The agent performing the action may appear in a "by the . . ." phrase or may be omitted.
(agent performing action has been omitted.)

Sometimes the use of passive voice can create awkward sentences, as in the last example above. Also, overuse of passive voice throughout an essay can cause your prose to seem flat and uninteresting. In scientific writing, however, passive voice is more readily accepted since using it allows one to write without using personal pronouns or the names of particular researchers as the subjects of sentences (see the third example above). This practice helps to create the appearance of an objective, fact-based discourse because writers can present research and conclusions without attributing them to particular agents. Instead, the writing appears to convey information that is not limited or biased by individual perspectives or personal interests.
You can recognize passive-voice expressions because the verb phrase will always include a form of be, such as am, is, was, were, are, or been. The presence of a be-verb, however, does not necessarily mean that the sentence is in passive voice. Another way to recognize passive-voice sentences is that they may include a "by the..." phrase after the verb; the agent performing the action, if named, is the object of the preposition in this phrase.