Typical components of a product requirements document (PRD) are:
Title & author information
Purpose and scope, from both a technical and business perspective
Stakeholder identification
Market assessment and target demographics
Product overview and use cases
Requirements, including
Assumptions
Constraints
Dependencies
High level workflow plans, timelines and milestones (more detail is defined through a project plan)
Evaluation plan and performance metrics
functional requirements (e.g. what a product should do)
usability requirements
technical requirements (e.g. security, network, platform, integration, client)
environmental requirements
support requirements
interaction requirements (e.g. how the product should work with other systems)
Not all PRDs have all of these components. In particular, PRDs for other types of products (manufactured goods, etc.) will eliminate the software-specific elements from the list above and may add in additional elements that pertain to their domain, e.g. manufacturing requirements.