Note: At ArcGIS 9.2, ArcSDE stopped being sold stand-alone. It is now ArcSDE technology and is included with both ArcGIS Desktop and ArcGIS Server.

ArcSDE technology is a core element of ArcGIS Server. Its primary role is to act as the database access engine to spatial data within a relational database management system (RDBMS). ArcSDE technology provides geospatial behavior, integrity, and utility to the underlying records of the geodatabase. The geodatabase is the primary data storage model for ArcGIS. It sits on top of the RDBMS and provides a single central location to access and manage spatial data.

Learn more about ArcGIS Sever.