The Autodesk Revit API is automatically installed with the default installation of the Autodesk Revit based product. Any .NET based application will reference the RevitAPI.dll and the RevitAPIUI.dll located in the Revit Program directory. The RevitAPI.dll contains methods used to access Revit's application, documents, elements and parameters at the database level. The RevitAPIUI.dll contains the interfaces related to manipulation and customization of the Revit user interface.
The Autodesk Revit API Software Development Kit (SDK) is installed from the Tools and Utilities section of the Autodesk Revit installation.