ArcObjects API
The Geodatabase API in ArcObjects offers fine-grained control over all types of geodatabases, as well as other types of GIS data. This API provides everything from simple data creation and querying, to the construction and use of advanced datasets (for example, geometric networks and topologies), as well as advanced geodatabase functionality such as versioning and replication. Using ArcObjects, developers can build stand-alone applications, as well as custom components that fully integrate into other Esri products such as ArcGIS Desktop. ArcObjects can be used with the .NET framework, Java, and COM-compliant languages such as C++.
ArcSDE API
The SDE API offers developers direct access to ArcSDE geodatabases. It differs from the ArcObjects API in that the programming objects deal with functionality at the ArcSDE tier of the software stack. The API allows for the creation, accessing, and querying of the tabular geodatabase data—including geometry and rasters—and the creation and modification of schema. SQL-level access is also provided to the Esri spatial type with operators allowing for the creation and querying of feature classes and their geometries at the SQL level.