When you use the <em><u>passive voice</u></em>, the subject is acted upon by the verb. In other hands, the action tends to be done <em>to the subject.</em>
<em />
<em>Sarah was </em><u><em>followed </em></u><em>by a ominous shadow.</em>
Sarah, the subject, is being acted upon.
The active voice describes a sentence where the subject performs the action stated by the verb. It is more clear and objective as well.
<em>An ominous shadow </em><u><em>followed</em></u><em> Sarah.</em>
Here, the subject, An ominous shadow, is acting upon Sarah.