CyanogenMod (pronounced sigh-AN-oh-jen-mod), is a customized, aftermarket firmware distribution for several Android devices. Based on the Android Open Source Project, CyanogenMod is designed to increase performance and reliability over Android-based ROMs released by vendors and carriers such as Google, T-Mobile, HTC, etc. CyanogenMod also offers a variety of features & enhancements that are not currently found in these versions of Android.
Right now, CyanogenMod consists of four parallel and active major versions: CyanogenMod 10 (Android 4.1), 10.1 (Android 4.2), 10.2 (Android 4.3) and 11 (Android 4.4). The variants of the firmware are split into categories, such as: Stable, Release Candidate, M-series and Nightlies.
The Stable version, as suggested by the title, is the tried and tested variant of the firmware proven to be mostly bug free and suitable for daily use. The latest stable version is available for an assortment of the officially supported devices.
A Release Candidate (RC) build may not be the final version, but a variant that has no fatal flaws or bugs, on the stabilization stages to become the final product that is the Stable variant.
The M-series releases behave similar to the RCs, but are considered ‘stable’ for our users.
Lastly there are the Nightlies, which are as volatile as a firmware can get. These releases keep coming at an interval of a day or two and if you do end up trying one of these, do not be alarmed if your device goes cuckoo on you. These ROMs are largely untested, and as advised by CyanogenMod, not meant for use for an average user. These releases, are meant to test untested waters that may or may not break your phone.