An update to YouTube has started rolling out in the Google Play Store.
The update, besides taking care of the usual bug fixes, brings some changes to the UI. There is a multi-window feature that lets you view a video in one window while the other window lets you search or go through your feeds.
Other additions include a hamburger style menu, video suggestion overlays during playback and the ability to open the guide to check on your subscriptions and feeds from anywhere in the menu. The HD and CC toggle has been moved from the left corner of the video to the menu and on 7 inch tablets, there is a fully-fuctional landscape UI.