What is "log"
Logs are essential to understand the activities of complex systems, especially in the case of applications with little user interaction (such as server applications).
Examples:
Examples of physical systems that have log subsystems include process control systems, and black box recorders installed on airplanes.
Many operating systems and complex computer programs include some form of log subsystem. In the simplest case, log messages are saved in a log file. Most operating systems and software structures also provide more sophisticated logging services. An example is the syslog service (described in RFC 3164), which allows filtering and recording log messages to be performed by a dedicated subsystem separately, rather than placing the onus on each application to provide its own ad-hoc registration system.
A server log is a log file (or multiple files) automatically created and maintained by an activity server exercised by it. A typical example is a web server log that keeps a history of page requests.
A audit log is a security-related record that provides documentary evidence of the sequence of activities that have affected at any time a specific operation, procedure or event.
Standards
Syslog
Syslog is an informal standard for computer data logging that was developed in the 1980s by Eric Allman. It was created exclusively for Sendmail, but proved so valuable that other applications started using it as well. Since then it has become the standard logging solution in Unix and Unix-like systems, there have also been a variety of implementations on other operating systems and is commonly found on network devices 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 common NCSA log format) and extended Log Format are standard text file formats used by web servers when generating log files. Because the formats are standardized, the generated files can be analyzed by a variety of analysis programs.
Common Log Format inputs take the form:
host ident authuser date request status bytes
Eg: 127.0.0.1 user-identifier frank [10/Oct/2000:13:55:36 -0700] "GET /apache_pb.gif HTTP/1.0" 200 2326