The difference between that and vanity publishing is quite simple.
Vanity is easier, cheaper and less time-consuming than printing a large amount of your own books. The quality of the binding is often (but not always) better.

That said, not all vanity publishing is onanistic. I know some genuinely good writers who used vanity publishing because their work was turned down by numerous printhouses because their topic wasn't quite what the market wanted at the time. Some of the books have been subsequently picked up by proper houses.

The biggest problem with vanity press is that there are absolutely no quality checks.