Private Label Rights are articles that are pre-written. People buy them and rewrite them - be it sentence by sentence or a few words.
Also known as PLR, these pre-written articles are available on a variety of topics. There are several PLR sellers. Some of them even specialize in certain niches. For instance, there are PLR sellers who specialize in internet marketing articles.
Regardless of their specialty, they all sell the same pack of articles to several buyers at low prices. This is why there's a concern for duplicate content, for PLR sellers sell the same package to everyone.
Hubs aren't PLR because these are original articles - unless some writers use PLR and rewrite it.
Content mills aren't considered private label rights articles. This is ghostwriting - hiring writers to write your content while putting your name on the content. And remember, content mills produce original content, not duplicate like PLR.
In short, when you use a content mill, you're using the services of a ghostwriter. When you buy PLR, you're buying content already written.