Contacts are relationships that exist in your Yahoo! address book, such as those entered from across the Yahoo! Network like Yahoo! Mail and Yahoo! Messenger. The API provides read and write access.
Social Directory describes a user's identity on Yahoo!, including their Yahoo! profile and relationships information. The API provides read and write access to public and private information.
The Yahoo! Application Platform enables developers to create and deploy their applications across the Yahoo! network of sites. It is based on, and supports, open standards including Open Social.