What is a Youtuber?
A youtuber is someone who uploads videos to Youtube, specially in a serious or professional manner, maintaining a channel with similar content following a niche to gain an audience of subscribers. The term contrasts to the typical Youtube user who either never uploads any videos, or simply uses Youtube for as a free video hosting service and isn't trying to become prolific in it as a video publishing platform.
An example of a youtuber is Linus Sebastian, whose channel, Linus Tech Tips (abbreviated LTT), is in the niche of tech tips. You can find youtubers in all sorts of niches: cooking youtubers, movie youtubers, book youtubers. I've seen a youtuber who just runs. Like on his feet. And he records himself running. And posts it on Youtube. He's youtuber.
A youtuber often is someone who earns money from Youtube, and they may be someone who pursued a "career" as a Youtuber. It's hard for me to actually call this a career since you can't really get promoted from Youtube. A Youtube user who meets some criteria in number of subscribers and views is eligible to monetize their channel, meaning they earn money from Youtube itself from the ads automatically shown in their videos. A youtuber doesn't need to meet this criteria to make money from ads. They can simply get a sponsorship somehow to advertise someone inside the video, then it's an ad, they earn money from it, even if they don't meet Youtube's criteria. It's possible for someone who knows how to be a Youtuber to seek a career in other platforms, like livestreaming on Twitch, or simply become an entertainer or something of sort, so there are transferable skills, it's just hard to say whether there is a career path in there considering that you are entirely dependent on one company, Youtube, giving you money. If they ban you, and they can ban you, you don't have a career anymore.
A youtuber may also be someone who isn't trying to earn money from Youtube videos, but simply posts them as a hobby. They became prolific because their videos are good. It's very likely that someone like this will still opt to monetize their channel once they meet the criteria because why not.