Primary interop assemblies (PIAs) are the official, vendor-supplied, .NET type definitions for interoperating with underlying COM types. Primary interop assemblies are strongly named by the COM library publisher to gurantee uniqueness.
ESRI provides primary interop assemblies for all the ArcObjects type libraries that are implemented with COM. ArcGIS .NET developers should use only these interop assemblies for interoperating with ArcObjects in .NET. The ESRI primary interop assemblies are installed in the Global Assembly Cache (GAC) during install if version 1.1 of the .NET Framework is detected.