Like everyone else has said, using the passive voice isn't "wrong;" it's just that most people prefer the active voice. Again, like everyone else has said, the active voice is more succinct and more direct. Usually the reader is more interested in who does something, rather than the object of that action.
However, there are cases in which the passive voice is rhetorically helpful--that is to say, more effective when making an argument. Let's say, for instance, that you're a prosecutor for a murder trial. The tactic you've chosen in making your cases involves painting a picture of the victim, characterizing the victim in such a way that the jury finds her sympathetic and is more likely to convict the defendant. You may start by describing the victim: "Sarah Marshall was a grade school teacher. She paid her taxes. She was a dog-walker. She was a mother of three." After you've described the victim, you have two ways to go about describing her death: the active voice ("He murdered her") or the passive voice ("She was murdered by him"). The active voice is fine, and even helpful, because it immediately connects the defendant to the act of the victim's murder and, in its brevity, it tonally communicates the brutality of the act. However, it also renders the victim--who, up to this point, was the subject of the argument--an object, thus placing her in a less rhetorically powerful position. The passive voice, in this case, highlights the tragedy of her death, because it places her as the primary focus of the sentence while immediately connecting her with a brutal death. In fact, because it's in the passive voice, the victim seems helpless; the murder happened to her, and she wasn't really involved in it. With the passive construction, the jury would keep the victim in mind, which (if the victim is particularly sympathetic) might help you win your case.