CyanogenMod is an aftermarket firmware distribution for smartphones and tablet PCs that run the Android operating system.
CyanogenMod is a heavily-tweaked system that also has the power to manipulate the hardware (such as the CPU clock) of Android devices. It also contains certain functionality and features that are not available in Android releases.
The firmware is compatible with dozens of Android-based devices, and can even be installed on older smartphones that have been rooted. The most recent CyanogenMod (v7.1) is based on Android 2.3 Gingerbread.
Due to a delay in the release of source code for Honeycomb, CyanogenMod was not released based on that iteration of Android. The next anticipated release of CyanogenMod will be v9, based on Ice Cream Sandwich.
The CyanogenMod website lists builds for specific Android devices and offers mods that are Stable, Release Candidate or Nightlies.
While only a few years ago it was required to root an Android smartphone (and act looked upon with disapproval) to install CyanogenMod, makers of smartphones now generally allow complete root access to their newer devices, meaning CyanogenMod can be flashed without the need to hack the device.