Photos is Apple's photo app that works across Macintosh, iOS, and iCloud. It replaces iPhoto and allows users to import, organize, edit, and share their photos across platforms. While similar to iPhoto in functionality, Photos removes some features like rating stars and iPhoto events. It introduces new sharing options and editing extensions from third parties. The document provides an outline and demonstration of Photos' capabilities for importing, organizing, editing, and publishing photos on Mac and iOS devices.