It's pretty close to Reddit, but it's like a whole bunch of smaller Reddits that share posts between each other. The individual Reddits (instances) are generally grouped by hobby, political affiliation, or geographical location, but not always.

You can tell what instance something or someone is from by looking at their name or title. E.g. I'm from lemmy.zip, you can see it after my user name.

The reason that it can't be bought by a billionaire is that even if one instance is bought out, compromised etc, it can just be removed from the group at large, and it continues like nothing happened. I'm simplifying a bit but I hope you get the general idea.