Node.js is a JavaScript runtime built on Chrome's V8 JavaScript engine. It allows developers to build server-side applications using JavaScript, which was previously only used for client-side development. Node.js is designed around an event-driven, non-blocking I/O model, making it efficient for building scalable and real-time applications.

The architecture of Node.js can be broken down into three main components:

1. V8 Engine: This is the JavaScript engine developed by Google and used in the Google Chrome web browser. It compiles JavaScript code into machine code and provides a high-performance environment for executing JavaScript code.

2. Libuv: This is a multi-platform library that provides an event loop and thread pool for Node.js, allowing it to handle multiple tasks concurrently. It provides an asynchronous API for handling I/O operations, making it an essential component of Node.js' non-blocking I/O model.

3. Node.js Core: This is the core of the Node.js runtime, and it provides a set of built-in modules and APIs that developers can use to build their applications. Some of the most commonly used modules include the HTTP module for building HTTP servers, the FS module for reading and writing files, and the net module for implementing network services.

Together, these three components form the foundation of the Node.js runtime, and developers can build their applications on top of this foundation, leveraging its high-performance, scalable, and flexible architecture.