Overview of SharePoint client-side web parts
SharePoint client-side web parts are controls that appear inside a SharePoint page but run locally in the browser. They're the building blocks of pages that appear on a SharePoint site.
You can build client-side web parts using modern script development tools and the SharePoint workbench (a development test surface), and you can deploy your client-side web parts to classic web part pages in Office 365 Developer tenants.
For more information, see the Getting started, Basics, and Concepts sections (in the TOC).