SQL Server Editions

SQL Server Compact Edition

An Embedded Database Engine. Lightweight, in-process database engine designed to run on devices and desktops and is geared toward local data storage.

SQL Server Express Edition

The entry point to the data service platform which runs on server and desktop versions of the Windows platform. Has the typical functionality of a data service engine, and has been extended to support usage as a local data store.

SQL Server Express does not come with the SQL Server Agent which is used in the full version to automate tasks, including backups.

SQL Server Workgroup Edition

Data service engine targeting small offices and branch offices. Adds additional functionality such as merge replication publishing, unlimited database size.

SQL Server Standard Edition

Data service engine targeting small and medium-sized organizations and includes entry point features for business intelligence.

SQL Server Enterprise Edition

Data service engine targeting large enterprises with advanced tools, analytics, data warehousing features, fault-tolerance and other enterprise-level features.

Requirements

http://msdn.microsoft.com/en-us/library/ms143506.aspx