TypeScript is a free and open-source high-level programming language developed by Microsoft that adds static typing with optional type annotations to JavaScript. It is designed for the development of large applications and transpiles to JavaScript. TypeScript is a superset of JavaScript, meaning that all JavaScript programs are syntactically valid TypeScript, but they can fail to type-check for safety reasons. TypeScript may be used to develop JavaScript applications for both client-side and server-side execution (as with Node.js or Deno).

TypeScript adds syntax on top of JavaScript, allowing developers to add types. It is a strongly typed programming language that builds on JavaScript, designed to give better tooling at any scale. TypeScript provides "type safety" to JavaScript, which means that it enables developers to add type safety to their projects. TypeScript allows specifying the types of data being passed around within the code, and has the ability to report errors when the types dont match. TypeScript uses compile-time type checking, which means that it checks if the specified types match before running the code, not while running the code.

Some of the key features of TypeScript include:

• Static typing: TypeScript comes with optional static typing and a type inference system, which means that a variable, declared with a certain type, cannot be reassigned to a value of another type.
• Interfaces: TypeScript supports interfaces, which are used to define contracts within your code and can be used to define the shape of an object, function, or class.
• Type aliases: TypeScript allows developers to create their own type aliases, which can be used to define custom types.
• Class-based object-oriented programming: TypeScript supports class-based object-oriented programming, which is a programming paradigm based on the concept of "objects", which can contain data and code to manipulate that data.

In summary, TypeScript is a programming language that builds on top of JavaScript, adding static typing and other features to enable better tooling and type safety for large-scale applications.