In an active voice construction, the subject is performing the action. In a passive voice construction, the subject is being acted upon. Although some students believe they should never use the passive voice, there are circumstances in which it is the most appropriate choice.

Overview: this brief overview from Purdue OWL explains when and why to use active voice over passive voice, accompanied by illustrative examples. 

Writing in an Academic Style Video: this useful UWC-produced resource explores a number of style features in academic writing, including (at 3:41) how to tell the difference between active and passive voice, and when to use each. 

Passive Voice: this comprehensive guide busts the myths of passive voice, describes strategies, explains when it is appropriate to use, and more.