Demystifying the WUSA.EXE (Windows Update Stand-alone) process and .MSU file format

A good KB article describing how to extract .MSU files, the files they contain, and how to use the contents with WUSA.EXE.

A quick rundown of the process:
When you use Wusa.exe to install an update package, Wusa.exe expands the contents of the .msu file to a temporary folder. Then, Wusa.exe performs the following steps:

1. Wusa.exe uses the Windows Update metadata in the .msu file to search for applicable updates.
2. Wusa.exe copies the contents of the applicable updates to the Windows Update sandbox. The Windows Update sandbox is a protected folder.
3. Wusa.exe calls the appropriate function in the Windows Update Agent API according to the mode in which you start Wusa.exe.
4. When the Windows Update wizard completes the installation, the Windows Update Agent API returns a status. Then, Wusa.exe works synchronously with Windows Update Agent API.

Read more about it here.