A deposition is an event at which attorneys ask questions to a witness or to the opposing party. The questions and the answers to those questions are recorded by a court reporter. The court reporter prepares a written transcript which is known as the "deposition transcript" but is commonly referred to as "the deposition".
A deposition takes place in person or via video, and sometimes it is filmed -- particularly if the person being deposed (the witness) is terminally ill. Showing a video deposition to a jury is more effective than reading a deposition transcript. (Deposition questions may be submitted in writing, but this is rarely done.)
Interrogatories are written questions submitted by one party to the other party who answers the questions in writing.
ORIGINAL ANSWER: The first main difference between an interrogatory and a deposition is who answers. In an interrogatory, the opposing party is questioned; whereas, in a deposition, a third party, or "potential witness" is questioned.
The second main usual difference between an interrogatory and a deposition is how the questions are asked. In an interrogatory, the questions are written, and answered on paper, still under oath. In a deposition, a lawyer questions the witnesses out
loud, and of course, under oath. However, deposition questions can also be presented in written format in some cases.