One analogy would be the relationship that exists between Etsy and its Sellers; Etsy is a website that hosts sellers' websites.
Another analogy would be between Blogger/Blogspot and its bloggers; Blogger/Blogspot hosts people's blogs.
In both cases above (and as with Ning), hostees are somewhat limited in what they can and cannot do with the website, but in return they can have their website in a hosted environment where much of the maintenance and coding is handled by the host.
So, using this analogy, Ning is a website that hosts social networks, of which Indiepublic is one.
A social network, btw, is something like MySpace, Facebook or Friendster.