Design documents are detailed plans that outline the specifications and requirements for a project, often used in fields like software development and engineering. They serve as a guide for the development team, ensuring everyone understands the goals, features, and constraints of the project. These documents typically include diagrams, user stories, and technical specifications, helping to communicate ideas clearly. By providing a structured approach, design documents facilitate collaboration among team members and stakeholders, making it easier to track progress and make informed decisions throughout the project lifecycle.