An authentication framework that handles authorization with external web services.
Check out the Keyring Developer's Guide.
See Keyring Social Importers for an example. You can also extend Keyring Service classes directly, rather than attaching the service as a property to an object (like the Importers do).
Keyring requires PHP 5.3+ to work, because it makes use of some modern features in PHP like late static binding and abstract classes. Other than that, as long as you meet the minimum required WP version, you should be OK to get started. If you get a cryptic "T_PAAMAYIM_NEKUDOTAYIM" error, you need to upgrade to PHP 5.3+.
Your webserver will also need to be able to make outbound HTTPS requests for some operations with some services to work correctly.
Most services within Keyring require some sort of API key/secret before you can connect to them.
Keyring just provides a framework for handling connections to external services. You need to download another plugin which makes use of Keyring to do anything useful (e.g. an importer or content-syncing plugin).
Add files to includes/services/extended/ that either implement one of the includes/services/core/ service foundations, or start from scratch. Follow one of the existing service definitions for a template, and see service.php in the root of Keyring for some detail on methods you need to define, and optional ones that might make your life easier.