Logging (software)
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 (i.e., 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.

Many operating systems, software frameworks and programs include a logging system. A widely used logging standard is syslog, defined in Internet Engineering Task Force (IETF) RFC 5424). The syslog standard enables a dedicated, standardized subsystem to generate, filter, record, and analyze log messages. This relieves software developers of having to design and code their own ad hoc logging systems.

Event logs 

Event logs record events taking place in the execution of a system in order to provide an audit trail that can be used to understand the activity of the system and to diagnose problems. They are essential to understand the activities of complex systems, particularly in the case of applications with little user interaction (such as server applications).

It can also be useful to combine log file entries from multiple sources. This approach, in combination with statistical analysis, may yield correlations between seemingly unrelated events on different servers. Other solutions employ network-wide querying and reporting.

Transaction logs 

Most database systems maintain some kind of transaction log, which are not mainly intended as an audit trail for later analysis, and are not intended to be human-readable. These logs record changes to the stored data to allow the database to recover from crashes or other data errors and maintain the stored data in a consistent state. Thus, database systems usually have both general event logs and transaction logs.

Transaction log analysis 
The use of data stored in transaction logs of Web search engines, Intranets, and Web sites can provide valuable insight into understanding the information-searching process of online searchers. This understanding can enlighten information system design, interface development, and devising the information architecture for content collections.

Message logs 

Internet Relay Chat (IRC), instant messaging (IM) programs, peer-to-peer file sharing clients with chat functions, and multiplayer games (especially MMORPGs) commonly have the ability to automatically save textual communication, both public (IRC channel/IM conference/MMO public/party chat messages) and private chat between users, as message logs.
Message logs are almost universally plain text files, but IM and VoIP clients (which support textual chat, e.g. Skype) might save them in HTML files or in a custom format to ease reading or enable encryption.

Internet Relay Chat (IRC) 

In the case of IRC software, message logs often include system/server messages and entries related to channel and user changes (e.g. topic change, user joins/exits/kicks/bans, nickname changes, user status changes), making them more like a combined message/event log of the channel in question, but such a log isn't comparable to a true IRC server event log, because it only records user-visible events for the time frame the user spent being connected to a certain channel.

Instant messaging 

Instant messaging and VoIP clients often offer the chance to store encrypted logs to enhance the user's privacy. These logs require a password to be decrypted and viewed, and they are often handled by their respective writing application..

Server log
A server log is a log file (or several files) automatically created and maintained by a server consisting of a list of activities it performed.

A typical example is a web server log which maintains a history of page requests.  The W3C maintains a standard format (the Common Log Format) for web server log files, but other proprietary formats exist.  More recent entries are typically appended to the end of the file.  Information about the request, including client IP address, request date/time, page requested, HTTP code, bytes served, user agent, and referrer are typically added.  This data can be combined into a single file, or separated into distinct logs, such as an access log, error log, or referrer log. However, server logs typically do not collect user-specific information.

These files are usually not accessible to general Internet users, only to the webmaster or other administrative person of an Internet service.  A statistical analysis of the server log may be used to examine traffic patterns by time of day, day of week, referrer, or user agent.  Efficient web site administration, adequate hosting resources and the fine tuning of sales efforts can be aided by analysis of the web server logs.

See also 
 Common Log Format
 Digital traces
 Event Viewer
 Java Logging Frameworks – logging frameworks for Java
 .NET Logging Frameworks - logging frameworks for .NET
 List of web analytics software
 Logbook
 Log4J
 Log management and intelligence
 Logging as a Service (LaaS)
 Pantheios – logging API library
 Runtime intelligence
 Server log
 Web counter
 Web log analysis software
 XML log

References 

Data logging
Computer logging