CyanogenMod is a custom ROM based on various versions of the Android operating system.
Have you checked the Cyanogenmod Wiki?
Taken from the CyanogenMod website:
CyanogenMod is an aftermarket firmware for a number of cell phones based on the open-source Android operating system. It offers features not found in the official Android based firmwares of vendors of these cell phones.
CyanogenMod's supported devices list boasts more than 160 mobile devices and their variants, including phones, phablets, tablets, and other hardware.
Major version numbers parallel vanilla Android (often referred to as AOSP) in the following manner:
- CyanogenMod 6: Android 2.2 (Froyo)
- CyanogenMod 7: Android 2.3 (Gingerbread)
- CyanogenMod 9: Android 4.0 (Ice Cream Sandwich)
- CyanogenMod 10: Android 4.1 (Jelly Bean)
- CyanogenMod 10.1: Android 4.2 (Jelly Bean)
- CyanogenMod 10.2: Android 4.3 (Jelly Bean)
- CyanogenMod 11: Android 4.4 (KitKat)
- CyanogenMod 12: Android 5.0 (Lollipop)
- CyanogenMod 12.1: Android 5.1 (Lollipop)
CyanogenMod 8 was reserved for Android 3.x (Honeycomb). Since the source code of Android 3.x was not made public until after Android 4.0 source was released, there was no CyanogenMod 8 release or development, because all development efforts went towards CyanogenMod 9.
Note that not all devices will necessarily receive official support from CyanogenMod. CyanogenMod is maintained by volunteers in their free time, so support depends on having enough interested developers. Also, support depends on compatible drivers being available for the device, so some phones will never be supported.
Similarly, devices may be dropped from further development for various reasons as new versions of Android are released (for example, the latest "stable" build for the Nexus One is CyanogenMod 7.2).
There are several types of CyanogenMod build. Nightly builds are created each day, and represent a snapshot of development, with minimal testing and quality control. They often have missing features or critical defects, changing from one day to the next, and are not recommended for everyday use. If you ask a question here about unusual behaviour in a nightly CM build, we'll probably suggest you switch to a more stable ROM. Experimental builds are the same as nightlies, but are one-off special builds outside of the nightly routine.
Milestone snapshot (a.k.a. M snapshot) builds are more stable than a nightly, because they're only built when the code is believed to be working, but they're still minimally tested, and may still have big problems.
Release candidates are undergoing the last testing before being designated as a stable release. They should be safe for everyday use, and it's very unusual to find serious problems, but they may still have odd bugs that just haven't been noticed yet.
Stable releases have "all or nearly all" issues resolved, and should be as stable as (or more stable than) an official, stock ROM. They go through rigorous testing, including by end users who've tried out the release candidate, and are recommended for everyday use.