Figma is cloud-based, so files live at at URL, not on disk. Multiple users can edit a file simultaneously, and will see their colleagues' cursors in real time. Versions of the file are being continuously saved, and files can be rolled back to previous versions if necessary.
UI design mockups and prototypes are created using an interface similar to Sketch. The concept of "components" is similar to Sketch's "symbols" with some additional flexibility.
Files and prototype can be shared in multiple ways. A file can be shared (with specific view/edit permission options) with colleagues. A URL can be created to allow anybody to access a file from a browser. An iframe can embed the live Figma file directly in a web page.
Colleagues can leave notes directly on the design files, with reply threads connected to each comment.
A [plug-in architecture](https://www.figma.com/plugin-docs/intro/) allows code to be written to add automation and other features to Figma. Plugins can be shared privately within an organization or published to the public community of Figma users.