OpenSocial is a set of common application programming interfaces (APIs) for web-based social network applications, developed by Google along with MySpace. A number of other social networks Applications implementing the OpenSocial APIs will be inter-operable with any social network system that supports them, including features on sites such as Hi5.com, MySpace, Orkut, Netlog, Sonico.com, Friendster, Ning, and Yahoo!

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.

The web is more interesting when you can build apps that easily interact with your friends and colleagues. 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.

OpenSocial defines a common API for social applications across multiple websites. With standard JavaScript and HTML, developers can create applications that access social networking sites and update feeds.

A common API means you have less to learn to build for multiple websites. OpenSocial is currently being developed by a broad set of members of the web community. The ultimate goal is for any social website to be able to implement the API and host 3rd party social applications. There are many websites that support OpenSocial, including hi5, LinkedIn, MySpace, Netlog, Ning, orkut, and Yahoo! It provides many sites on one API.

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 breviloquent explanation:
An Opensocial application made by You-At is Locomi. It makes it simple for you to find the best places to eat, shop and have fun in your city. You can use Locomi to review the places you like and the ones you don't, keep a tab on interesting events and see what your friends recommend. Locomi is available on Facebook and Orkut and is coming soon to your mobile.

Happy discovering!