Answer Description

System logs will show every event on an operating system. This is very useful to find if some one accessed the system, among many other things.

Wikipedia

In computing, a log file is a file that records either events that occur in an operating system or other software runs, or messages between different users of a communication software Logging is the act of keeping a log In the simplest case, messages are written to a single log file A transaction log is a file (ie, log) of the communications between a system and the users of that system, or a data collection method that automatically captures the type, content, or time of transactions made by a person from a terminal with that system For Web searching, a transaction log is an electronic record of interactions that have occurred during a searching episode between a Web search engine and users searching for information on that Web search engine