What is OpenSocial?

In a nutshell, OpenSocial is a programming standard that allows mini applications to be built and embedded within a social network; these applications can have access to profile information, friend data (social graph), and recent activities data from the network. Here’s a clear and succinct explanation:

OpenSocial allows developers to write a single backend for their application and then have it distributed widely, across various social networks, where it can adapt an appropriate look and feel and take advantage of the data provided by the network container. For social network users, OpenSocial means that a richer and more varied set of applications will soon be available for them to deploy on their profile pages.