As soon as the kernel and the processes start executing and communicating with each other, a lot of information is produced. Most of it is sent to files - the so-called log files or, simply, logs.
Without logging, searching for an event that happened on a server would give sysadmins many a headache, hence the importance of having a standardized and centralized way of keeping track of any system events.
Traditionally, system messages have been managed by the standard logging facility — syslog — or any of its derivatives — syslog-ng or rsyslog. The logging daemon collects messages from other services and programs and stores them in log files, typically under /var/log. However, some services take care of their own logs (like Apache HTTPD web servers). Likewise, the Linux kernel uses an in-memory ring buffer for storing its log messages.
Log files in /var/log
Because logs are data that varies over time, they are normally found in /var/log. Here's a list of which logs are found there: