Red Hat Enterprise Linux
Red Hat Enterprise Linux, or RHEL as it's more commonly known (because it's a long name), has a very firm footing in Enterprises. It targets the commercial space very well, and as a result it is not uncommon to find yourself on a RHEL box, which you've initially assumed to be a CentOS installation.
What makes RHEL different is the support offered by Red Hat, Inc. and the various services that you can utilize if you've bought an official package.
While Red Hat still offers the source code for their distribution without question (hence CentOS), they sell versions and packages for everything from the desktop up to data centre installations.
There's an adage that states "no one got fired for buying IBM," which is a little dated in this day and age, but I've heard people invoke this philosophy to describe Red Hat on more than one occasion. No one will get fired for buying Red Hat (but you might be asked what the benefits are of paying for something that's available for free under another name.)
Aside from the support, the business-like attitude that other businesses like, and the contributions back to the community as a whole, Red Hat also offer something which has been variously described as "a waste of time" and "crucial for this role."
Exams are looked at with both affection and derision, depending on who you speak to in the Linux community (as with many things, there's something of a holy war about them). Red Hat offers two of the most popular, and many more to boot. You can study for and become a Red Hat Certified System Administrator, followed by a Red Hat Certified Engineer, which are widely seen as very acceptable qualifications to have.
As a college dropout, I'm quite happy to have an RHCE qualification under my belt.
Some people see these exams as a way of getting past the first line of those hiring (as in the people who scan your CV, and look for badges they recognize). Others see them as proof that you know what you're doing around a Linux system due to the fact that these exams are practical, (meaning they sit you in front of a computer and give you a set of steps to finish.) Some people dismiss exams altogether, although they're usually the ones who've never bothered trying the exam.