Nginx, pronounced as engine x, is a lightweight HTTP server. It also has modules that allow it to work as a reverse proxy and mail server without installing any other dependencies. You can find out more information by visiting http://wiki.nginx.org/.
The Raspberry Pi does not have too much RAM to spare, and nginx is specially designed to use the least amount of resources but deliver content at higher speeds. You can tweak Apache, but it is still very bulky; this will not be discussed in this book because even the best optimized Apache server is not as good as the normal nginx installation.
Nginx streamlines input and output processes using the shortest route possible, where Apache relies on other modules to do the work, even if it is to serve a simple file. These modules in Apache take up unnecessary RAM and CPU power.
