Verbs have two voices: active voice and passive voice. "Voice" should not be confused with tense. Tense has to do with the time of the action; voice pertains to the way a verb functions relative to the subject of the sentence.
With active voice, the action of the verb is linear and straightforward. The movement of the sentence is subject –> verb –> rest of idea. "The report [subject] summarizes [verb] our findings [object and rest of idea]" is such a sentence.
With passive voice, the verb acts upon the subject. What would be the subject in a sentence using an active verb becomes the object. "Our findings are summarized by the report" is such a sentence.
Another way to put this is: When the verb is active, the subject performs the action; when the verb is passive, the subject is the recipient of the action.
In the examples given above, both verbs are present tense; the difference between them is voice. This has a subtle effect upon the impact of the sentence. The active voice ("The report summarizes our findings") is more direct and forceful. The passive voice ("Our findings are summarized by the report") is less direct and requires more words. One reason for this, of course, is that the passive voice uses a helping verb: are summarized.
An elementary rule of style is: Prefer the active voice. That is, use the active voice unless you have strong reason to use the passive voice. (Notes regarding when the passive voice may be used appear after the active/passive examples below.)
Here are some examples of active (A) versus passive (P) in the six main tenses:
. . . in present tense
A The article discusses the effects of unemployment.
P The effects of unemployment are discussed in the article.
. . . in present perfect tense
A The class has decided that everyone should get an "A."
P It has been decided by the class that everyone should get an "A."
. . . in past tense
A The company made a huge profit.
P A huge profit was made by the company.
. . . in past perfect tense
A By Tuesday, we had reached an agreement.
P By Tuesday, an agreement had been reached [by us].
. . . in future tense
A We will mail our proposal next week.
P Our proposal will be mailed next week.
. . . in future perfect tense
A We will have finished the job by next month.
P The job will have been finished [by us] by next month.
In the progressive (a.k.a. continuous) tenses, the passive voice is especially burdensome because the progressive tenses already use auxiliary verbs to which the passive voice adds another auxiliary. Thus, the active present progressive "I am doing my homework" is, in its passive form, "My homework is being done by me." Ouch!
Partly because electronic grammar checkers diligently point out passive verbs (it is one thing they can be programmed to do easily), some people have begun to think that passive vouce is flat-out incorrect. While active voice is preferable when possible, passive voice is often necessary, sometimes unavoidable, and occasionally desirable.
Here are situations in which passive voice is desirable [derived from Theodore Bernstein: The Careful Writer. (New York: Atheneum) 1979]:
> When the agent performing the action is considered unimportant or too obvious to mention. "McGregor was accused of murder": Who did the accusing is unimportant. "The mail has already been delivered": Obviously, the mailman delivered it.
> When the performer of the action is indefinite or unknown: "Silk hats are not worn these days." "These legends were first recorded in the sixth century."
> When we want to emphasize the doer or the thing done by placing it at the end of the sentence: "The new system was marketed by Microsoft" (emphasizes doer). "We can't drive because our car is being repaired" (emphasizes thing done).
> When we want to tone down a statement, to make it less direct. "We believe that certain positive steps need to be taken . . . ." "It has been suggested that . . . ."