From MEPIS Documentation Wiki
The typical system produces a number of files which need to be preserved in runtime, even through a system reboot. Examples include log files, print job spool files, mail spool files and the like. These are almost always placed in the /var filesystem.
In addition, applications that share data for multiple users typically place their files under /var. For example, the anonymous FTP directories are often under /var/ftp, while the Apache web server files are commonly placed under /var/http. Database applications like MySQL also commonly place their data files under /var.
The /var directory contains files that are constantly changing while Linux is running. Logs (/var/log), system mail (/var/mail) and queued processes (/var/spool) are stored in this directory.