
What are the limitations of SQL Server Express?
I've been asked this question a few times, in different context. It is interesting since it is after all free. So SQL Server Express Edition is a scaled down, free edition of SQL Server, which includes the core database engine.
While there are no limitations on the number of databases or users supported, it is limited to using one processor, 1 GB memory and 4 GB database files. The entire database is stored in a single .mdf file, and thus making it suitable for XCOPY deployment. It is intended as a replacement for MSDE. Two additional SQL Server 2008 versions provide a superset of features not in the original Express Edition. The first is SQL Server Express with Tools, which includes SQL Server Management Studio. It is a free, easy-to-use graphical management tool for configuring, managing, and administering SQL Server 2008 Express. It can also manage instances of the SQL Server Database Engine created by any edition of SQL Server 2008. SQL Server Express with Advanced Services adds full-text search capability and reporting services.
Get more information at the Wiki and at Microsoft.