Julian Assange's organization had highly sensitive US State Department cables on its servers. These are cables that have not been released to the general public because they contain sensitive material that could get people killed. The cables are the raw material, no names redacted. Well, now they're loose because Assange trusted someone he knew well enough to give them access and the guy made off with them and now it's all available online, names and everything.
Politico says "The encrypted file contains over 250,000 diplomatic cables that in many cases identify diplomatic sources, including activists, journalists and academics in dictatorial societies."