SharePoint 2013 apps authentication with OAuth

SharePoint 2013 provides a new method for application authentication. Despite the term authentication, it is based on OAuth 2.0 (Open Authorization). The idea is that an app can access other parts of SharePoint without asking users credentials, but using user's permissions in a frame of selected authorization that user accepts during initial app installation. The apps access requirements are build in apps manifest by the developer. Refer to technet article and this video.