What's CentOS other than number 15 on DistroWatch's list of popular Linux distributions? Officially, CentOS is an "Enterprise-class Linux Distribution derived from sources freely provided to the public by a prominent North American Enterprise Linux vendor. CentOS conforms fully with the upstream vendors redistribution policy and aims to be 100% binary compatible. (CentOS mainly changes packages to remove upstream vendor branding and artwork.)."
What that really means is that CentOS is built from Red Hat's RHEL (Red Hat Enterprise Linux) source code, which is freely available at the Raleigh, NC company's ftp site. It's positioned as a free or low-cost server alternative to RHEL 5.
In short, it's the RHEL for expert Linux users who don't require Red Hat's support contracts. Mind you, there are also CentOS support companies, but CentOS' real market is businesses that don't need hand-holding.
As the CTO of a major West coast Web site told me at OSCon, "We have a large commercial Web site serving tens of millions of page views a month with lots of advertising revenue. We like RHEL, and of course we looked at Red Hat. However, Red Hat's subscription prices, at about a grand per server per year, were too much for us. We just didn't see enough value in paying them for support since we already had experts in-house."
Another reason for CentOS' data-center popularity is that CentOS is easy to set up as a server. I use it myself on my own Web servers. I find it easy to maintain, easier to manage, and very fast.
There you have the CentOS story in short. It's not just Linux-savvy Web sites, though, that have adopted CentOS as their favorite flavor of Linux. Oracle, to create its Unbreakable Linux, an RHEL clone, uses CentOS as its template.
So, if you already have a lot of Linux expertise at your fingertips and want to run some serious Web servers, give CentOS a try. It's not for everyone -- there's a reason why Red Hat does so well with its RHEL subscriptions -- but for Linux experts, CentOS is a worthy Linux server alternative.