Update, 27th October 2009: Please see Software architecture document guidelines for an updated version of the guidelines.
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. My simplified view of the content included in a software architecture document is :
During our tutorial last week at QCon, we asked attendees to define the software architecture for a small software system and provided a handout containing some guidelines. Since this may prove useful for other people, we're making Software Architecture Document Guidelines v0.1 available for download.
It's worth remembering that the software architecture doesn't have to be a huge weighty tome and it doesn't even need to be a traditional Word document. What's important is that you capture the important architectural decisions and principles. This set of guidelines includes suggestions for what you might want to include. Just as a final note, this set of guidelines is a work in progress and we'll be iterating it over the coming months. Any feedback is welcome.