What is a Requirement?
The requirements are the high-level descriptions about a particular system services, constraints or to a detailed specification that are generated during the requirements gathering process.
User Requirements - It is a detailed description in natural language along with diagrams of the services the system provides and its operational constraints. It is usually developed by end users.
System requirements - It is a structured document detailing the descriptions of the system's functions, services and operational constraints.
Functional Requirements - It describes the services of the system, how the system should react to particular inputs and how the system should behave in definite situations.
Non-functional Requirements - It describes the attributes of the system.
Domain Requirements - Requirements that arises from the domain of the application and that reflect characteristics of that domain. It can be either functional or non-functional specifications.
Requirement Document Structure:
User requirements definition
System requirements specification