ArcSDE technology is integrated into ArcGIS Desktop, ArcGIS Server, and ArcGIS Engine so that users can scale their DBMS-based geodatabases across their organizations.
ArcGIS Desktop and ArcGIS Engine
ArcEditor, ArcInfo, and ArcGIS Engine, ship with a copy of Microsoft SQL Server Express that can be installed with these products. You can use ArcCatalog to set up and administer an ArcSDE geodatabase in SQL Server Express.
ArcGIS Server Workgroup
This level of ArcSDE supports simultaneous editing by small workgroups, people on many projects and web-based editing applications. In order to support this, ArcSDE and Microsoft SQL Server Express are included in ArcGIS Server Workgroup. This enables support for ArcSDE geodatabases in smaller server configurations.
ArcGIS Server Enterprise
This includes full ArcSDE capabilities that can scale to large numbers of users and very large geodatabases. You are responsible for installing and administering your own DBMS. Supported DBMS's include: DB2, Informix, Oracle, PostgreSQL, and SQL Server.
See About new installations of the ArcSDE component for a summary of the steps needed to set up an ArcSDE geodatabase licensed through ArcGIS Server Enterprise.