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 firmware available from the vendors of these cell phones.
CyanogenMod is currently is based on Android 2.3 Gingerbread and referred to as CyanogenMod 7. CyanogenMod offers the most barebone Android experience coupled with some very powerful tweaks. This whole package by now is not wholly developed by CyanogenMod developers alone, but is a collaborative effort between them and independent developers on sites like XDA-Developers.