Nginx is a free, open-source, high-performance HTTP server and reverse proxy, as well as an IMAP/POP3 proxy server. it is known for its high performance, stability, rich feature set, simple configuration, and low resource consumption.
Nginx is one of a handful of servers written to address the C10K problem. Unlike traditional servers, Nginx doesn't rely on threads to handle requests. Instead it uses a much more scalable event-driven (asynchronous) architecture. This architecture uses small, but more importantly, predictable amounts of memory under load.