Whats the difference between a deposition and an interrogatory?

There is a fundamental difference between an interrogatory and a deposition and both of them serve different purposes.

An interrogatory, also known as a Request for Further Information, is a set of written questions posed to the opposite party that must be answered truthfully, in writing, under the penalty of perjury.

How ever on the other hand, deposition is a particularly useful device that may be used at trial to present first-hand witness testimony in the event that the witness becomes unavailable.