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/spool/: contains the working files of the system's daemons. 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).
/var/run/: used to keep track of all processes utilized by the system, enabling you to act on them in the event of a system change runlevel (see Chapter 11, The Start-Up Files: init sysv).