RHN is to RHEL what "WSUS" is to Windows. It's just the mechanism used to push updates out to clients and for those clients to pull new packages/updates down from. It presents to the client systems via a yum plugin and so from the system administrator's point of view all the packages available on RHEL just look like they're coming from a regular yum repository.
You need a subscription to do RHEL updates, but it's not strictly required. If you're only concerned with system update, CentOS might be a better fit. Regardless of what they say, they do change stuff around (sometimes beneficially) and they include extra packages that don't have the same level of QA that Red Hat puts into their RPM's. You also lose out on some of the features you get from the web interface to RHN where you can review and schedule updates, group them, etc. You also lose out on web/phone support and if a bug is creating a sev1 issue for you, then you're on your own.
But for the packages common to both distributions, CentOS is 90-95% repackaged and rebranded RHEL. If your goal is just system updates and none of the other stuff, that's what you should be looking at.
For which one is more common, it's anyone's guess. No one really tracks hard numbers on that stuff. Having a RHEL subscription is obviously ideal so you do get the extra stuff (makes managing many systems much easier) but CentOS is also designed for people who are budget-centered and can self-support. RHEL is pretty ubiquitous in high end data centers, but there are lots of mom and pop shops that use CentOS. It's kind of hard to tell which out weighs the other (install base-wise).