Users with karma over 200 (I think) have a 'flag' link. If multiple users click it then a submittion or comment goes [dead]. In your user profile you can opt-in to see those, the default is to hide them. I flag a couple per day, usually SEO spam ("lock smith in city X"), gambling sides and other promotions and the occassional story that doesn't belong, let's say "Famous footballer has an affair".
The moderators, that's two people afaik, have their own self-build tools and more visibility. Well, they have access to the servers and databases. They can ban users permanently, change algorithms, merge stories, change titles and such. Nobody else is moderator.