Nginx is a web server that is known for its high performance, low resource usage, and ability to handle large amounts of traffic. It is commonly used as a reverse proxy, load balancer, and HTTP cache. Nginx is also popular for its ability to serve static content quickly and efficiently. Its modular architecture allows for easy customization and extensibility through the use of third-party modules. Nginx is open source software and is available for free under the BSD-like license.