SQL Server Express is a free, easy to use, redistributable version of SQL Server 2005 designed for building simple data-driven applications. SQL Server Express has most of the features offered by SQL Server full version, but with certain limitations. This makes SQL Server Express a good choice for small and medium scale applications.
Most of the limitations of SQL Server Express edition will not affect the small/medium scale applications.
Limitations of SQL Server Express Edition
1. Number of CPUs supported
SQL Server Express uses only one CPU at a time. It can be installed on a server with multiple CPUs, but it will use only one CPU at a time.
2. Maximum memory used
SQL Server Express uses a maximum of 1 GB memory for it's data buffer. So, if your server has severaql GB memeory, SQL Server Express cannot take advantage of it.
4. Database size limit
Maximum database size is limited to 4 GB (log file size is not counted)
5. Profiler in SQL Server Express
Profiler tool is not included with SQL Server Express editions.
6. Job Scheduler
Job Scheduling service is not available with SQL Server Express.
Data import and export feature is not available with SQL Server Express.
A complete feature comparison of various editions of SQL Server can be found at the Microsoft web site - http://www.microsoft.com/sql/prodinfo/features/compare-features.mspx