MIUI is probably the single biggest ROM in all of China, and the biggest reason for this is that it ships with Xiaomi's phones. Xiaomi, if you somehow haven't heard of them, is one of the fastest growing vendors in all of Android, and at this point is only under Samsung and Lenovo in China in terms of volume shipped as of Q1 2014. MIUI is a custom version of Android made by Xiaomi, and was long popular before the phone manufacturer ever really became popular for its phones. MIUI is sort of a hybrid of iOS, Android, HTC's Sense Android skin and sports a number of unique features that have pushed it to be one of the leading custom ROMs out there.
V4 debuted with Android 4.0 Ice Cream Sandwich, and V5 has made its way through all of Android Jellybean from 4.1 to 4.3, and the latest releases even are based on Android 4.4 KitKat. MIUI v6 promises to be another big user interface overhaul, including a rumored mess of new features to play with in addition to the new UI. MIUI v5 looks sort of like HTC's Sense 4.0 in many ways with the stock skin, but seeing as theming is a huge part of MIUI the stock look will probably not stay that way once you get into the themes store and start shopping around for yourself.
MIUI v6 aims to change your desire for simply moving to another theme as soon as you launch the device. Featuring light gray colors as a base and a color changing notification bar, as has become popular recently and officially adopted into Android as of this fall's Android L release, MIUI v6 looks to change the notification bar's color based on what's present at the top of the screen. Other than that we're seeing plenty of UI placement changes, flattening all over, and a generally simplified visual look. MIUI v6 is still based on Android 4.4 Kitkat, specifically Android 4.4.4, so you won't be getting any Android L specific goodies with this just yet. The expected release date is sometime after the official unveiling on August 16th.