Here is the what Cyanogen mod is:
Cyanogenmod is a custom ROM made by developers (or you may say freelancers) and it is an opensource GPL licensed software. Actually cyanogenmod is modified version of actual android software released by Google. Developers at Cyanogenmod make software specific to various devices.
And if you want to install Cyanogenmod you will have to root your phone and flash it with Cyanogenmod.
This will surely void your warranty. Cyanogenmod is like installing a clean new android software and it is quite fast than Stock one, as it does not contain any useless apps pre installed and also it is capable of using phone's processor to its full level which is often limited in Stock ROM's.
I am using it on my Motorola Defy and it works fine, though there are some bugs but its okay.
Note: Installing Cyanogenmod without following correct procedure may brick your phone (i.e. make it permanently unusable).
I would recommend you to read their wiki for more info.