Your question asks about a subject that could take up a whole semester at law school to provide an answer. Generally, "discovery" is the means by which parties to a case request and obtain from each other the following information: documentary and physical evidence, pre-trial statements and testimony, and any other relevant matters in the custody, possession or control of the other. There is a whole set of procedural rules which govern how, when and what discovery may be had. Discovery most commonly consists of the following: a list of questions called interrogatories which one side asks the other to answer; a request for the production of documents and other tangible evidence in the form of a list describing the categories and types of items which one party wants to obtain from the other; a written list of factual statements which one party asks the other party to either admit or deny; and depositions, which are recorded testimony of a party or witness under oath transcribed by a court reporter under questioning by the other party. There are other forms of discovery, and failure on one party to respond to properly requested discovery as required by the rules may subject that party to sanctions by the court, including dismissal of a claim or other relief. It is not a process that a non-lawyer is likely able to navigate effectively. Many cases are won and lost in discovery. It is an important tool for establishing the factual grounds necessary to prove or defend your case, and in limiting what the opposing party can prove at trial. If not pursued effectively, you risk losing your case.