The new SDK is built on top of the Guzzle HTTP client framework, which provides increased performance and enables event-driven customization. Each AWS service client extends the Guzzle client and describes operations on the service using a service description file. The SDK now manages persistent connections for both serial and parallel requests. It detects transient network failures, with automatic retries using truncated exponential backoff. Support for event hooks (via the Symfony2 EventDispatcher) allows you to implement custom, event-driven behavior.
In the AWS post about the update, they give you a few code snippets showing this updated version in use. This completely reworked version of the SDK is not compatible with the previous version, so you'll need to consult their migration guide to bring things up to date.