"Objection your honor, that's hearsay!"
We've all heard it, either in court or on tv. But what does it mean? Well, law schools spend weeks on hearsay and all of its many exceptions. But let's try to narrow it down to something workable for you - or at least workable 90% of the time. The actual definition of hearsay is "an out of court statement that is offered for the truth of the matter asserted."
Now do you see why lawyers drink?
So let's forget the actual definition and focus on what it means and then boil it down to one concept. And that concept is this - when you are testifying in court you are not allowed to state what someone else told you. So if you are on the stand and the lawyer asks you, "Well how do you know that?" If your answer is, "Because Sally told me," then you are providing hearsay testimony and it will likely be objected to. Remember that you can only testify to matters of which you have personal, first-hand knowledge.
Another important thing to remember is that if you have someone that is helpful to your case then he / she needs to make arrangements to be at your hearing. Many times clients will want to get a letter from this person, even have it notarized, and then use that at the hearing. No can do kemosabe. Documents can be hearsay as well. So if that person has valuable information that helps your case then he / she needs to be at your hearing to testify. Think of this situation in reverse and you'll understand why: You're at your trial / hearing and your ex's attorney gives the judge a letter from Sally and the attorney says that Sally is the model human being and has never done anything wrong in her life and has never lied in her life and the letter speaks glowingly about your ex and that fine, upstanding citizen that he is. As you push the vomit back down your throat, you want the judge to know that Sally is a scumbag and that she goes to fundraisers without paying for a ticket! But alas, Sally is not present! All we have is this stupid letter! Never fear my dear, hearsay to the rescue! The letter is hearsay because it cannot be cross examined by you or your lawyer. If they want Sally's statement then the scumbag needs come front and center and testify. And once she's done with her blather, then your lawyer can get up and point out why she is not as credible as she seems.
So, to sum it up, don't say things on the stand that other people said or testify about things that other people told you. Also, if you need information at your trial / hearing from a particular person - that person needs to be at your trial / hearing.
Remember, this is 90% of the time. If you have any questions, ask your lawyer!