The /var directory contains all operating 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 log files;
/var/spool: holds the system daemons' working files. For example, /var/spool/lpd contains the print server's working files while /var/spool/mail holds the e-mail server's working files (i.e.: 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 "The Start-Up Files: init sysv").