Angular (aka Angular 4) is an open source web framework built by Google to develop high performance single page applications (SPA). Google officially announced the latest Angular version, known as Angular 4, in 2016. It's completely rewritten, and doesn't resemble its previous version.
The Angular 4 documentation is extensive and in depth; you can read through it at https://angular.io/docs/ts/latest/.
The Angular 4 framework is written with TypeScript, its recommended language for writing web components for Angular. So what is TypeScript, then?