Passive voice refers to sentences that use a form of the verb "to be" and in which an action occurs but the identity of the person or object performing the action (the subject) is unclear or simply not emphasized. Passive voice always puts the emphasis on the object receiving the action, as opposed to the subject. Sometimes this is the right choice, but writers usually try to avoid the passive voice when possible because the passive voice often leads to wordier and less powerful sentences. Instead, writers prefer the active voice because the subject of the sentence is actually doing the action (thus making the sentence more commanding and confident).
Remember, in most cases you want to be “active” not “passive” in your writing.
Passive Voice: The midterm exams were graded by the instructor.
These steps help spot a passive sentence and revise it into the active voice:
Side Note: Imagine that you are the director of your sentence, and you have paid your star (a.k.a. the subject of your sentence) a lot of money to act in your sentence. You want them to perform for you.
Active Voice: The instructor graded the midterm exams.
Side Note: It's impossible to completely avoid the use of "to be," but if you find that most of your sentences are built around "is," "are," "were," etc., it's a sign that the majority of your writing is passive and could benefit from being made more active.
For more about writing actively and concisely, see the Writing Concisely page.