You may want to use Shazmo's consise references to Federal Law and State Law . Discovery generally takes place informally outside of direct court supervision. Parties must meet and confer to maintain a steady course of discovery. Parties are also required to supplment earlier production. The scope of subject matter in controversy may widen over the course of litigation.
Relevance is hard to determine before production.
Contrary to liberal discovery is a concern that discovery will be used as an instrument of coercion. Non-parties can't be burdened with demanding travel. Judge can conduct in camera review to weight for himself the sensitivity of documents subject to discovery request. Documents can be redacted to protect the identities of private associations etc. The purpose of discovery is to avoid trial by ambush.