Nginx is an OPEN-SOURCE Reverse Proxy Server for HTTP, HTTPS, SMTP, POP3 AND IMAP as well as a LOAD BALANCER, HTTP cache and a Web-server.
Nginx is successful attracting the traffic because the way it handles the requests is very different from apache web-server.
Nginx generates thread for each connection which means it will occupy huge space but its not true as it uses asynchronous event-driven handler for requests. In simple words connection cone-in treated independently, they can share memory space.
Which leads to more accurately provide predictable performance under very little loads.