A product requirements document (PRD) is a document containing all the requirements to a certain product. It is written to allow people to understand what a product should do. A PRD should, however, generally avoid anticipating or defining how the product will do it in order to later allow interface designers and engineers to use their expertise to provide the optimal solution to the requirements.
Typically, a PRD is created from a user's point-of-view by a user/client. The requirements are then analyzed by a (potential) maker/supplier from a more technical point of view, broken down and detailed.