Is MIUI just another version of CYanogen Mod?
As far as I can tell MIUI is a ROM developed by Xiaomi Inc and was designed to mimic iOS more than other android ROMs. It has been available to install on many devices for a while as well as for their own brand of high end Chinese Android devices (http://www.mi.com/en/).

As far as I know, its only relation to CyanogenMod is that MIUI used the core backbone of CyanogenMod to build upon in creating its own ROM and so will share some commonalities/similarities but they have always been available separately and often compared to each other as 'competition' choices with people favouring one of the other for their differences in overall design in the end product.