Here are some resources for those who want to learn more about the SWF format.

• Alexis' SWF Reference - An extensive guide to the SWF format based on the (apparently defunct) OpenSWF.org documentation.
• Macromedia Flash File Format - Another (somewhat outdated) guide the SWF format.
• Macromedia SWF Specification License - The official SWF specification is available here, although only to licensed users.