SQL Server Express is a fully featured database management system from Microsoft. It is a free download. There are no catches, it is free.
The tables in SQL Server link to an Access database. After linking, they behave in exactly the same was as normal Access tables, so why use SQL Server? The simple answer is security. Since 2007 MS Access has only implemented a single password sign on. This mean that anyone using the database can literally do anything with the data (including deleting the lot!) Whilst this might be acceptable for small and trusted groups it does not fit with the usual requirement of a database system used in a busy office by a multitude of people. There needs, at the very least, to be a check on who can delete data. Fortunately SQL Server security is as good as the ‘old’ Access 2003 security because is allows you to fine tune who can do what to the data.
There are other benefits around data-security. With an Access database all the data exists in a single file that can be copied and hacked into, Admittedly, when Microsoft changed the security system in Access 2007 and later versions it implemented a data encryption algorithm that went quite a long way to making it a secure system but at the end of the day all you have to do it steal the file and then you can take as long as you like to crack the password.
SQL Server, on the other hand, keeps its data in tables that are in folders that only the database administrator can access. We can also create the database so that the users cannot see these tables. This means that users can only look at the data presented by queries, forms and reports that the database administrator wants them to see.