The /var directory contains all operative data for programs running on the system. Unlike the working data in /tmp, this data must be kept intact in the event of a reboot. There are many subdirectories, and some are very useful:
/var/log: contains the system's log files which you may read to troubleshoot your system (/var/log/messages and /var/log/kernel/errors to only name two).
/var/run: used to keep track of all processes utilized by the system since it was booted, enabling you to act on them in the event of a system change runlevel (see Chapter 11, The Start-Up Files: init sysv).
/var/spool: contains the system's working files waiting for some kind of action or processing. For example, /var/spool/cups contains the print server's working files, while /var/spool/mail contains the mail server's working files (for example, all mail arriving on and leaving your system).