If your YouTube app looks different, you're not imagining it. Google's just done a major design and feature refresh across YouTube apps as well as the desktop.
Keeping you watching, and watching
Of course it's obvious the YouTube logo's gotten a tweak as well as the YouTube icon. Apart from that, the desktop version now has what Google says is a redesign based on Material Design - it's cleaner and rather fresh though not a radical departure from the original. There's also a Dark Theme, which turns the background dark for a more cinematic feel.
On the YouTube app, videos will now automatically adapt to however you're viewing it, with no annoying black bars on the side. Whether vertical or horizontal, the app will display the video at its optimum. You also now get gestures - double-tap on the left or right to rewind or fastforward a video. Soon to come is swiping to switch between videos.
You can also now choose to tweak the viewing speed of a video on mobile, a once desktop-only feature. Besides that, a list of suggested videos will also display at the bottom when you're watching videos in full-screen mode.