CentOS is the professional choice for LAMP (Linux+Apache+MySQL+PHP) and other servers. It is very stable, secure and flexible. It is almost identical to Red Hat Enterprise Linux (RHEL) and it is binary compatible with it. CentOS is a 24/7 server OS frequently described as a "free enterprise class computing platform".
 Why you want CentOS
CentOS is a great OS for running LAMP-servers. It's got almost all the great features of RHEL and the only main difference is that CentOS is free while RHEL is expensive.
Fairly new desktop softwrae packages are also included (KDE, Gnome, etc) and official website story is that it's a good desktop OS alternative for enterprises as well as a great server distribution. However, nobody but nobody admits using it for desktops and I honestly can't see why anyone would want to.
Again, it's a great choice for host I-need-a-dedicated-SQL-server or I-want-to-host-a-ton-of-websites type usage. It's the default OS choice for dedicated servers at the U.S. ISP EZZI.
 Stable equals old
CentOS is stable, and part of the reason is that the packages are generally way older than packages included in bleeding edge distributions - only security updates and patches are added to the stable package set.
However, it is possible to use a seperate repository to get newer packages.
 CentOSPlus: PHP 5, etc
CentOS gives you LAMP with Apache 2.0.52, a MySQL 4.0.x version and PHP 4.3.x, which is nice since these packages are known to be rock solid stable.
However, there are reasons you may want a newer LAMP. And you can have it by using the optional CentOSPlus repository.
- Edit the file /etc/yum.repos.d/CentOS-Base.repo and change enabled=0 after the [centosplus] heading to enabled=1 to use the CentOSPlus packages.
yum will now include PHP v5.x - and other newer packages such as MySQL v5.x - to the package list used by yum install and yum update.
You may want to read the readme before enabling it.