A mission report, or mission log, is the record of a particular mission.
Mission reports usually contain data about the badfic that the assigned team of agents was sent into, what the agents did (more or less, as some things are undoubtedly left out), a record of any charges given, and the resulting action taken.
Due to the popularity of uploading mission reports to the Internet, where they can be seen (and are usually known as PPC stories) and have resulting fanfiction created, there were slight "problems," which eventually led to the creation of Fanfic Land in order to stop this strange bleedthrough process.