Microsoft SQL Server is a relational database management system developed by Microsoft. It was first launched in 1989 and continues to be popular to this day.
SQL stands for Structured Query Language and is used to communicate with the database. SQL Server fully supports this language and allows you to perform CRUD operations using INSERT, SELECT, UPDATE and DELETE statements.
SQL Server is popular at being used in conjuction with .NET applications and allows the two to work side by side, transfering data between the application and the database.
Other features include the use of profiler tools to debug usage, supporting multiple databases per server, and being able to support large amounts of data.