A YouTuber is a term for a person who has an account on YouTube and creates content. Unlike unregistered users, any YouTube channel has the capability to like, dislike, or comment on videos, subscribe to other channels, upload videos, etc.
YouTube celebrities are people or groups who are exponentially popular because of their videos on YouTube. Some of them have corporate sponsors, who pay for product placement in their clips or production of online ads. Some YouTubers have even become famous in the media. Canadian singer, Justin Bieber was discovered on YouTube and became one of the world's biggest superstars at only 15 years old.
YouTube Partners are YouTubers who make money off of the videos they make on YouTube. In the early days there was only YouTube/Google Adsense, which gave the uploader a little bit of money whenever a viewer clicked on the ads on their videos; however, as of 2009/2010, third party partnership programs have shown up to partner-specific YouTuber that makes specific content; although not all of them, many of these YouTubers have found a way to make a living off of their partnerships, which is known to be impossible with normal ad sense.
Users used to be able to post video responses underneath of people's videos, but as of 2011, YouTube got rid of that feature.