draw.io is a free online diagram software for making flowcharts, process diagrams, org charts, UML, ER diagrams, network diagrams and more. It supports a wide variety of diagram types and offers a user-friendly interface with drag-and-drop functionality. Diagrams can be saved in multiple formats, including PNG, JPEG, SVG, and PDF, and can be stored on various platforms like Google Drive, OneDrive, Dropbox, and locally.