Nginx (pronounced as “engine x”) is a high performance and a low memory usage web server. Apache is a process-based web server and nginx is an asynchronous web server.
As nginx wiki says,
“The main advantage of the asynchronous approach is scalability. In a process-based server, each simultaneous connection requires a thread which incurs significant overhead. An asynchronous server, on the other hand, is event-driven and handles requests in a single (or at least, very few) threads.