Apache is an open-source HTTP server implementation. It is the most popular web server on the Internet; the December 2005 Web Server Survey conducted by Netcraft shows that about 70% of the web sites on Internet are using Apache.
This is a step by step guide to installing memcached from source on CentOS. The version of CentOS that I am using is CentOS release 5.2 (Final); to find what version you are using: