Log data consists of files that are records of events, transactions, or messages:
Event logs collect actions that take place at a specific time (timestamp) that can be used to understand user behavior, application, and system activity, or to diagnose problems.
Transaction logs gather changes to stored data that allow you to recover the database from crashes or other data errors.
Message logs track (i.e. save) textual communication, both public (IRC channel/IM conference/MMO public/party chat messages) and private messages between users.