What is CentOS?

Basically if you want to learn Red Hat, you can download CentOS for free. You can download RHEL source for free, but unless you want to spend weeks building it, I suggest you download CentOS instead. CentOS is built from the RHEL source, and only the trademarks (logos) have been replaced.

CentOS 

CentOS (Community Enterprise Operating System) is a Linux distribution which attempts to provide a free enterprise class computing platform which has 100% binary compatibility with its upstream source, Red Hat Enterprise Linux (RHEL)

RHEL is available only through a paid subscription service that provides access to software updates and varying levels of technical support. The product is largely composed of software packages distributed under free software licenses and the source code for these packages is made public by Red Hat.

CentOS developers use Red Hat's source code to create a final product very similar to RHEL. Red Hat's branding and logos are changed because Red Hat does not allow them to be redistributed.

CentOS is available free of charge. Technical support is primarily provided by the community via official mailing lists, web forums, and chat rooms. The project is not affiliated with Red Hat thus receives no financial or logistical support from the company; instead, the CentOS Project relies on donations from users and organizational sponsors.

Details- http://www.centos.org/