When you use the active voice, you tell the reader directly who or what is doing the action. Sentences in active voice follow the pattern in which someone or something (noun) does something (verb). The do-er, the person or thing that does the action, is the subject of the verb. For instance…

“The boy threw the ball. It ricocheted off a lamppost and landed in the fountain with a splash that drenched a sleepy pigeon.”

Here the do-ers are specified: the boy, the ball, and the splash.

When the passive voice is used, the recipient, the person or thing the action is done to, is the subject of the verb. The do-er, if included, is there simply to modify the verb. Here’s what the above example would look like in the passive voice…

“The ball was thrown by the boy. A lampost was ricocheted off of by the ball. The fountain was landed on, and a sleepy pigeon was drenched by the splash.”

In this version, the subjects of the verbs are: the ball, the lampost, the fountain, and the pigeon. http://www.how-to-write-a-book-now.com/the-passive-voice.html 

Sentences in active voice are also more concise than those in passive voice because fewer words are required to express action in active voice than in passive.