What is SharePoint Framework

• SharePoint Framework (SPFx) is a client-side development model that allows us to create modern SharePoint experiences using modern web parts and UI extensions
• SPFx is available for SharePoint Online, 2019 and 2016
• SPFx is a page and web part model that provides full support for client-side SharePoint development, easy integration with SharePoint data, and support for open source tooling
• SharePoint Framework is the next evolution in SharePoint development
• SharePoint Framework is a new developer model accompanying the new SharePoint UX

Key features of the SharePoint Framework include the following:

• It runs in the context of the current user and connection in the browser. There are no iFrames for the customization (JavaScript is embedded directly to the page).
• The controls are rendered in the normal page DOM.
• The controls are responsive and accessible by nature.
• It enables the developer to access the lifecycle in addition to render, load, serialize and deserialize, configuration changes, and more.
• It is framework-agnostic. You can use any JavaScript framework that you like: React, Handlebars, Knockout, Angular, and more.
• The toolchain is based on common open source client development tools such as npm, TypeScript, Yeoman, webpack, and gulp.
• Performance is reliable.
• End users can use SPFx client-side solutions that are approved by the tenant administrators (or their delegates) on all sites, including self-service team, group, or personal sites.
• SPFx web parts can be added to both classic and modern pages.

Overview

See: https://docs.microsoft.com/en-us/sharepoint/dev/spfx/sharepoint-framework-overview