OpenSocial is the technology that drives orkut. OpenSocial is an initiative to build social networking sites around a central user account resource. With OpenSocial, companies like Google let you create a single user account that applies to several separate applications such as e-mail, calendars and instant message services. While single sign-on (SSO) lets you to log in to several applications with only one set of user credentials, OpenSocial goes beyond that by sharing your complete user profile between those applications, often in the same graphical user interface.
As an OpenSocial programmer, you don't have to start from scratch by setting up social gadgets. OpenSocial already has an application programming interface (API) with code for many common software actions used in a social network. For example, you can add a single line to your code to call a function from the API which requests all the extended profile information about a particular user. Then, you can use that information for other actions in your social gadget, such as to list the user's favorite books, or to show that user's location on a map. OpenSocial.org provides several articles and tutorials to help you get started creating your social gadget, with several examples featuring Google.