In their words:

Ning is a free online service (or, as we like to call it, a Playground) for people to build and run social applications. Social “apps” are web applications that enable anyone to match, transact, and communicate with other people.

As I understand it, it’s a service that lets you easily create “applications” like HotOrNot, KittenWar, Match.com, Craigslist and many others. I never thought these services were called social applications. Anyway it looks very interesting.

Ning is the (secret until now) product of 24 Hour Laundry, a startup by Marc Andreessen and others.