SPFx allows developers to build client-side SharePoint solutions using open-source tools like Node.js, Gulp and Yeoman. It uses TypeScript to build responsive and mobile-ready solutions that can run on both SharePoint Online and on-premises. Main components include Node.js for dependencies, Npmjs for managing them, Gulp for automating tasks, TypeScript for compiling code, and Yeoman for scaffolding solutions. Developers can use frameworks like React, Angular or Knockout and Microsoft Graph, and solutions are contributed to GitHub.