CentOS, which stands for the Community Enterprise Operating System, is a distribution of the Linux operating system based on RHEL (Red Hat Enterprise Linux). It runs on the x86 PAE (Physical Address Extension) and x86-64 architectures, and is currently the most widely-used Linux distribution for web servers.
CentOS uses the Red Hat source code base to create a product similar to RHEL. Unlike RHEL however, CentOS does not require a paid subscription. Product support is conducted entirely through community mailing lists, web forums, and chat rooms, and the developers are supported through donations and organizational sponsorships.