A .bak file is the ultimate format for backing up your data and provides you with all the data stored in your website.
Your website contains tables and tables of information.
When you pull things like membership reports on your website the report pulls information from these different tables to get you what you need.
The .bak files stores your SQL Server database up for you.
You should be able to restore .bak files from windows sql server 2008 into this free version of windows server.