Microsoft SQL Server is a relational database and is the one I use whenever an app requires a database.
SQL Servers are very complex pieces of software, at least as complex as an actual operating system.
For my work I use the SQL Server in a simplistic way; to store and serve data. Mostly the SQL Server is used in conjunction with a CMS, and in my case, Umbraco CMS. But I also use SQL for mobile apps and sometimes for bespoke websites that may have a blog section.
I would also use SQL for websites that need log in and log out features.
If you are interested in MS SQL then a good place to start finding out more about this technology is at their wiki entry:
Microsoft SQL Server: SQL Server