Passive voice is when the object of the verb becomes the subject. For example, "The documents were shredded." Or "Mistakes were made." What these sentences don't tell you is who shredded the documents or who made the mistakes. In most cases, use of the passive voice disguises the real issue, the point that you really want to make. But you can easily change the above sentences to the active voice: "She shredded the documents" and "I made a mistake."
The passive voice does have a place, however. When writing lab reports, particularly the methods section, you'll want to use the passive voice. For example, "The sodium chloride solution was transferred from a beaker to a test tube." The emphasis is on the solution, not the person who made the transfer.
When you write a sentence with the passive voice, ask yourself whether you want to emphasize the person responsible for the action or the object being acted upon. And if you find yourself writing a lot of "was" or "were," or "is" and "are," in your papers, try using more active verbs. A short list follows: