Microsoft's Windows operating system is written and developed in the USA, so it's no surprise North Korea doesn't really trust it given the tense relations between the two countries. Until 2002, when Red Star OS began to be developed in the DPRK, the few available computers in North Korea generally ran Windows. (Interestingly enough, the North Korean hackers which seem to be behind the hack of Sony Pictures appear to have been using Windows PCs instead.)
Other countries--especially ones very wary of U.S. influence--have also chafed at Windows. China once developed Red Flag Linux, but that project was shut down in 2014. Russia and Iran have also looked to Linux's open-source code as a customizable alternative to Windows.
But Red Star OS isn't just designed to shield North Korea from potential U.S. government backdoors. The Korea Times, a South Korean paper, reported that the operating system "is mainly designed to monitor the Web behavior of its citizens and control information made available to them." It's not clear if Red Star OS could do this outside of North Korea, but I don't recommend taking that chance.
It's like a normal Linux distribution, but...
North Korea's Red Star OS feels a bit like a Linux distribution from a bizarro world. There's a lot that's very recognizable, but it's been twisted in ways that feel unfamiliar.
Red Star OS is a Linux distribution with a customized, themed KDE 3 desktop. Previous versions of Red Star OS looked more similar to Windows, but the current Red Star OS 3.0 has a Mac OS X-like theme complete with a dock. A photo of Kim Jong-un in 2013 showed him with an iMac on his desk, so the reason for this redesign could simply be because the country's "dear leader" picked up a Mac and liked the interface.
Along with the KDE 3.0 desktop, North Korea's official browser of choice is Mozilla Firefox, which is renamed "Naenara," which translates to "my country." It's used to access a web portal also named Naenara, a country-wide intranet with limited access to the outside Internet.
The operating system uses North Korea's Juche calendar, which was introduced in 1997. The first recorded day in history is Kim Il-sung's date of birth on April 15, 1912. 1912 was year 1. This means that 2014 is year 103 in Red Star OS.
Red Star OS bundles various other utilities and games you'd recognize from a traditional Linux distribution, including the Wine compatibility layer for running WIndows software. The only language available is Korean, with North Korean spelling. The wallpapers folder contains various photoshopped background images designed to function as propaganda, according to Fast Company.
This is not for you
Red Star OS isn't made to be shared with the outside world. We only know about the operating system because we've gotten occasional snapshots of it from visitors to North Korea.
Russia's RT news network covered Red Star back in 2010 after a Russian student returned from North Korea with it, showing off a fairly standard looking KDE 3 desktop. In late 2013, Will Scott picked up a copy of it from a Korea Computer Center retailer while visiting Pyongyang to give a lecture at a university. That's where the news about--and screenshots of--the Mac OS X makeover came from.
It's not hard to find ISO images of Red Star OS that have somehow leaked to the outside world, though. Copies of Red Star OS 3.0 and earlier versions are floating around torrent sites. No, I'm not going to link to them here. You probably shouldn't run software developed by the North Korean government on your computer.
Interestingly enough, Red Star OS 3.0 seems to require a serial key to run. Perhaps North Korea isn't happy its operating system is being "pirated" online!
Red Star OS is an oddity. It's easy to be amused at from afar, noticing how the operating system became a Mac clone after the country's dictator got his hands on a Mac. But it's also a Linux distribution designed to filter and monitor web usage, created by perhaps the most oppressive government on the planet. It's both sad and interesting to see Linux twisted into such a system--although it's an allowed use of open-source software. (North Korea probably wouldn't give you the source code to Red Star OS if you asked, though.)