Computer data logging is the process of recording events in a computer program, usually with a certain scope, in order to provide an audit trail that can be used to understand the activity of the system and to diagnose problems. Logs are essential to understand the activities of complex systems, particularly in the case of applications with little user interaction (such as server applications).
Examples of physical systems which have logging subsystems include process control systems, and black box recorders installed in aircraft.
Many operating systems and complex computer programs include some form of logging subsystem. In the simplest case, log messages are written to a log file. Most operating systems and software frameworks also provide more sophisticated services for logging. One example is the syslog service (described in RFC 3164), which allows the filtering and recording of log messages to be performed by a separate dedicated subsystem, rather than placing the onus on each application to provide its own ad hoc logging system.
A server log is a log file (or several files) automatically created and maintained by a server of activity performed by it. A typical example is a web server log which maintains a history of page requests.
An audit log is a security-related log that provides documentary evidence of the sequence of activities that have affected at any time a specific operation, procedure, or event.
Syslog is an informal standard for computer data logging that was developed in the 1980s by Eric Allman. It was created solely for Sendmail but proved so valuable that other applications began using it as well. It has since become the standard logging solution on Unix and Unix-like systems; there have also been a variety of implementations on other operating systems and it is commonly found in network devices such as routers.
The Internet Engineering Task Force has documented (but not formalized) the standard in RFC 5424.
Common Log Format
The Common Log Format (also known as the NCSA Common log format) and Extended Log Format are standardized text file formats used by web servers when generating log files. Because the formats are standardized, the files generated may be analyzed by a variety of web analysis programs.
Common Log Format entries take the form:
host ident authuser date request status bytes
127.0.0.1 user-identifier frank [10/Oct/2000:13:55:36 -0700] "GET /apache_pb.gif HTTP/1.0" 200 2326
Courtesy of SO tag logging