What exactly is deposition testimony?

Deposition testimony is - more or less - the answers that are given during informal hearings before the trial. In general, deposition is part of the discovery process where lawyers on both sides have the opportunity to learn more about who potential witnesses are, what they do, and what they can contribute that's relevant to the case (if anything at all - after all, deposition hearings can also be used to weed out witnesses that aren't going to be able to further either side's case). As the court reporter, you'll be the only court official at most depositions - there won't be a judge or any sort of jury present - and it will be up to you to ensure that there's an accurate transcript available of the deposition testimony that you witness. The transcripts that you create will be distributed to everyone involved in the trial, may be used as evidence in the case, and can even be admitted if - once the case goes to trial - a particular witness is unable to attend the hearing to testify. As for what you can expect when you witness and transcribe deposition testimony, that can vary a bit. Sometimes you'll be in a lawyer's office, other times you'll be in other unofficial locations. There may be audio recordings made or there may be video equipment. A lot of the time, you'll find that it depends on what the case is about and how it's likely to move forward.

You've got the basics down right - deposition testimony happens before the trial not as a part of it. As a result, if you're going to be covering a deposition, one of the first things to expect is that the process may not even be taking place in a courtroom. Even though lawyers for both the prosecution and the defense will be present, chances are good that there won't be a judge attending - and you can rest assured that there won't be a jury on hand. Beyond that, the process of gathering a deposition testimony is about two things. First, it gives the lawyers a sense of how credible a witness will be when they are on the stand and whether or not the person being questioned should be used as a witness in the case - whether or not they have good, relevant information to report. Second, it helps the lawyers on both sides get a sense of whether or not the case that they are pursuing should even go to court in the first place.