Microsoft SQL Server Express is a cut down version of Microsoft SQL Server . However, it is completely free. Why? It acts as an introduction to the full version of SQL Server and this is a win-win for developers and Microsoft.
If is also used in embedded applications where you need something with the capabilities of SQL Server but want the application (front-end) and SQL Server (back-end) on the same machine.
Even though it is cut down this is still a very powerful database manager. It still has most of the same advantages over Microsoft Access as the full version of SQL Server, however it does required the expensive infrastructure or IT support.