Initially released in 1989, SQL Server is available with a commercial license. It does have one free version, SQL Server Express, with a limitation of 10 GB per database, along with other resource limitations. SQL Server is usually installed on Windows, but can also be installed on Linux. SQL Server is a great choice for organizations that need a good RDMS, and use a lot of other Microsoft products.

The advantages of SQL Server are that it offers a lot of functionality, including replication, and high availability and partitioning works very well with other Microsoft products, such as .NET Framework and Visual Studio. It is also fast and stable. 

The disadvantages of SQL Server are that licensing is expensive, especially for the Enterprise edition, and not all features are included in all editions, such as some high-availability options and partitioning.