A YouTube app comes pre-installed on most smartphones. However, the new iOS 6 will kick the built-in app to the curb. In order to watch YouTube videos on iOS 6 gadgets, you'll need to download the official YouTube app.
Not only does it have all the features you're used to, but more videos and a new, sleek interface. YouTube promises a bunch of new features to work with iOS 6, so stay tuned for more updates.
I'm also including a link to the Android version in case your smartphone or tablet is missing it.