Rule 26. General Provisions Governing Discovery; Duty of Disclosure
<2> The most substantial rule, which guides the discovery process.
Subdivision (a) provides for automatic disclosure, which first was added in 1993. Disclosure requires parties to share their own supporting evidence without being requested to by the other party. Failure to do so can preclude that evidence from being used at trial. This applies only to evidence that supports their own case, not anything that would harm their case. For example, a plaintiff brings a case alleging a negligent accident where the defendant damaged the plaintiff's boat. The plaintiff would then be required to automatically disclose repair bills for his damaged property (Since this would only support his case) (26(a)(1)(c)).
Subdivision (b) is the heart of the discovery rule, and defines what is discoverable and what is limited. Anything that is relevant is available for the other party to request, as long as it is not privileged or otherwise protected. Under §1, relevance is defined as anything more or less likely to prove a fact that affects the outcome of the claim. It does not have to be admissible in court as long as it could reasonably lead to admissible evidence.
However, there are limits to discovery. §2 allows the court to alter the limits of discovery on the number of depositions, interrogatories, and document requests, if it determines that the discovery sought is overly burdensome, redundant, unnecessary, or disproportionately difficult to produce with respect to the importance of the case or specific issue. Enshrined in §3, the work-product doctrine protects tangible (and some intangible) items created in anticipation of the litigation (e.g., a memorandum from an attorney outlining his strategy in the case). Protecting work product is considered in the interest of justice because discovery of such work product would expose an attorney's complete legal strategy before trial. §4 allows discovery of experts whose opinions may be presented at trial, but limits discovery of experts not likely to testify during trial. §5 generally prohibits the discovery of any material legally privileged (attorney-client, doctor-patient, etc.), and requires the production of a "privilege log" which describes the privileged information or material in a way that allows others to see that (if) it is privileged, but does not divulge the privileged material.
http://en.wikipedia.org/wiki/Discovery_(law)