http://www.dotnetfunda.com/interviews/exclusive/show/939/what-is-the-main-difference-between-access-and-sql-server

SQL Server is a more robust database management system. SQL Server was designed to have many hundreds, or even thousands of users accessing it at any point in time. Microsoft Access on the other hand, doesn't handle this type of load very well. If you do think you need to upgrade to SQL Server, don't panic - it's not a scary as you might think! And you didn't just waste your time learning Access. You will find that SQL Server is similar in many ways to Access. Now that you know your way around Access, you should be able to relate to SQL Server much better.