The Aura framework is available at https://github.com/forcedotcom/aura. Note that the open source Aura framework has features and components that are not currently available in the Lightning Component framework. We are working to surface more of these features and components for Salesforce developers.
The sample code in this guide uses out-of-the-box components from the Aura framework, such as aura:iteration and ui:button. The aura namespace contains components to simplify your app logic, and the ui namespace contains components for user interface elements like buttons and input fields. The force namespace contains components specific to Salesforce.