Regardless of the development process that you use, a description of the software architecture can be essential for any project, big or small. If software architecture is about the structure of a system and is the vehicle for satisfying the requirements, then the software architecture document is a written description of this.
The Software Architecture Document Guidelines v0.1 includes:
- An outline description of the software architecture, including major software components and their interactions.
- A common understanding of the architectural principles used during design and implementation.
- A description of the hardware and software platforms on which the system is built and deployed.
- Explicit justification of how the architecture meets the non-functional requirements.
Simon Brown has made the Software Architecture Document Guidelines (PDF) available for download.
Source: Coding Architecture