There are two main types of applications that can be developed with the Orkut platform:
On-site applications, also called gadgets, are the applications that run embedded in the Orkut user interface. Users can search for them through the Application Directory and can add them to their profiles. These applications must go through an approval process before they are available to users.
Off-site application is an application that runs outside of Orkut and communicates with the platform over HTTP. These applications do not appear in the Orkut directory and users cannot "install" them or add them to their profiles. In fact, users don't need to visit Orkut at all in order to interact with an off-site application: they can interact directly with your site or device.