Where to start to understand a unknown database
Your best bet to start is to document your database using SQL Power Doc

SQL Server &amp; Windows Documentation Using Windows PowerShell
SQL Power Doc is a collection of Windows PowerShell scripts and modules that discover, document, and diagnose SQL Server instances and their underlying Windows OS &amp; machine configurations. SQL Power Doc works with all versions of SQL Server from SQL Server 2000 through 2014, and all versions of Windows Server and consumer Windows Operating Systems from Windows 2000 and Windows XP through Windows Server 2012 R2 and Windows 8. SQL Power Doc is also capable of documenting Windows Azure SQL Databases.

Note: I have used it and it will give you a really good start in documenting and understanding your database server instance.