Urban Dictionary describes it best:
you have just entered the very heart, soul, and life force of the internet. this is a place beyond sanity, wild and untamed. there is nothing new here. "new" content on 4chan is not found; it is created from old material. every interesting, offensive, shocking, or debate inspiring topic youve seen elsewhere has been posted here ad infinitum. we are the reason for "not safe ". we are the anonymous army. cross us and you will fail. anonymous is everywhere. you depend on us every day. we bag your groceries, we fix your computers. anonymous sees you before you see him. sitting at desks around the world right now is a nameless, faceless, unforgiving mafia composed of the best of the best.