Discovery is the legal process by which both parties to a lawsuit are able to gather the pertinent information in order to litigate or defend a legal claim. It takes place before the trial so that both parties to the dispute have the facts they need.
Discovery takes many forms. One way to obtain information through discovery is to send the other party a list of written questions, which is called an interrogatory. Another way is a deposition, in which you give sworn testimony that is recorded by a court reporter. Or, if there are specific documents you know that you need, you can request those items directly.
For state courts, state law dictates the precise rules that govern discovery. Federal law governs discovery in federal courts.