Fairphone launched the Fairphone 2 earlier this year, and developers that owned the phone were able to throw on the in-development, open source OS that the company was working on over at code.fairphone.com. Now the OS has been released for everyone to have fun with. This OS from Fairphone is open sourced and it is based off of Android 5.1 Lollipop. However according to the blog post that Fairphone put up on their site, the open source OS doesn't include Google Mobile Services. This means no Google Play Store, Google Maps, or anything Google related. Essentially, it could be seen as a forked version of Android.
The blog post also mentions that if you choose to install the operating system on your Fairphone 2, it will delete all of your data. So you'll want to be sure to back up your phone ahead of time, or you'll lose that data forever. Fairphone mentions that the open source version of their software is going to be getting updates as often as their regular OS with Google Services. However, they did mention that there may be some time difference between updates, with the Fairphone OS with Google services likely getting updated first, followed by the Open Source version.
You can think of this as something similar to Cyanogen. Although a bit backwards. Since CyanogenMod was first, and was open sourced (to an extent) being based off of AOSP. Then later they formed Cyanogen Inc. to do Cyanogen OS which is proprietary and closed source. Right now it looks like the Fairphone open source OS only works on the Fairphone 2. Which Fairphone did also mention that they made the smartphone a bit modular so that you are able to fix the device should anything need replacing. With iFixit having made a repair guide, and Fairphone selling spare parts for the Fairphone 2. It's a pretty neat concept and it's going to be interesting to see how things work out for the company with the Fairphone 2. Fairphone's core value is transparency. And that's exactly what putting out a open source operating system shows, considering that the code is out there for anyone to virtually anything with.