In this discussion you can report possible spam. Also, if some spam was found and deleted, it's announced here for you to see what's going on. In case something in the gray zone was deleted you can post statements about it here.
I'd say it's about posts with sole intent to advertise/promote something with little informative value to the members of the network. But I don't want to get to exact with the definition. When in doubt, specific cases can be discussed here before deletion.