TypeScript is an open-source, object-oriented programming language developed and maintained by Microsoft. It is a typed superset of JavaScript and it compiles to plain JavaScript. 

Broadly speaking, TypeScript is JavaScript with some additional features. These include classes, modules, and interfaces to support the development of large-scale JavaScript applications. It provides a type system for JavaScript making it easier to build complex applications with large teams. The main benefit of TypeScript is static type-checking – highlighting errors while typing the code without running it. Since it uses the same syntax and semantics as JavaScript, TypeScript is also easy to learn.

Since TypeScript compiles to JavaScript, it can be used for both client-side and server-side execution (as with Node.js), with any browser and any operating system just like JavaScript. This also means that any existing JavaScript code is also TypeScript code. It works with existing JavaScript frameworks and libraries without any issues. In fact, Angular is based on TypeScript.

Find out more about it on its homepage.