When a lawyer says "objection" during court, he is telling the judge that he thinks his opponent violated a rule of procedure. The judge's ruling determines what the jury is allowed to consider when deciding the verdict of a case.

What are the rules of evidence?

The rules of evidence govern what may and may not be considered when the jury decides the outcome of a case. While there are many rules of evidence, they generally can be reduced to just a few principles:

• Witnesses may only present facts that they personally observed. This seems pretty obvious, since testimony would be useless if witnesses were allowed to just say anything that comes to mind. However, in application this is a bit tricky. A witness can say "I saw the blue car drive through a red light before hitting the pedestrian," but a witness cannot say something like "The driver of the blue car should go to jail because he ran a red light and hurt someone," because it is the witness's opinion that the driver should go to jail. Also, lawyers are not allowed to ask leading questions, such as "Where did the blue car go through the red light?" because it suggests to the witness that this event occurred.
• Every witness must be able to be cross-examined. Cross examination is the part of trial when one attorney tries to discover lies or other problems with a witness's testimony. The right to cross-examine stems from the 6th Amendment right to confront your accuser, and is there to ensure that every piece of testimony is rigorously examined before going to a jury. This is why "hearsay testimony," or testimony about what some else told the witness, is generally not allowed -- the other person is not there to be cross examined. However, there are exceptions to this rule.
• Documents must be authentic. Sometimes, the parties seek to put documents or other items into evidence, and there are many evidence rules in place to make sure that the item in evidence is the original evidence, or at least an accurate copy.

How does a judge rule on objections?

A judge can rule one of two ways: she can either "overrule" the objection or "sustain" it. When an objection is overruled it means that the evidence is properly admitted to the court, and the trial can proceed. When an objection is sustained, the lawyer must rephrase the question or otherwise address the issue with the evidence to ensure that the jury only hears properly admitted evidence. In theory, the jury should even disregard the improper question asked, although this can be difficult to do.

An objection is important to procedure even if it is overruled. Once a lawyer objects to some evidence, that objection is on the record. If the lawyer disagrees with the judge's ruling, he can then appeal that decision. If the lawyer failed to object to evidence he loses the right to appeal, even if the evidence was admitted improperly.

See FindLaw's section on Courtroom Procedure for more information.