The /var directory contains all the operating data for programs running on the system. Unlike the working data in /tmp, this data must be kept intact on the event of a reboot. There are many subdirectories, and some are very useful:
/var/log: contains the system log files;
/var/spool: contains the system daemons working files. For example, /var/spool/lpd contains the print server working files and /var/spool/mail the e-mail server working files (i.e. all mail arriving on and leaving your system).
/var/run: this directory is used to keep track of all processes being used by the system, so that you can act on these processes in the event of a change of system runlevel (see chapter "The startup files: init "System V"").