The new tmpfs-mounted
/run folder allows programs like udev, lvm and mdadm to keep runtime data from initrd until shutdown.
/var is a standard directory of any Linux/UNIX system - it stands for "variable" and is a place where a lot of logs, cahces, BUT also program variable settings files and even some system configuration databases reside.
Most things in
/var should be properly purged and regulated by the system. Your swap files for virtual memory also live in
/var so don't mess with that.
/var/run also holds a lot status and parameter information of actively running process daemans.
This directory contains system information data describing the system since it was booted. Files under this directory must be cleared (removed or truncated as appropriate) at the beginning of the boot process. Programs may have a sub-directory of
/var/run; this is encouraged for programs that use more than one run-time file.
/var/run is mounted as tmpfs. That means it's totally empty when your machine boots and it's meant to be like this to prevent stuff like daemons not starting because of a left-over PID-file.
Startup scripts usually create the directories they need before using them. If you want to store a PID-file either put it in
/var/run directly or create a directory before creating the PID-file. This is no place to store data that needs to remain there across reboots.
Sources:Pathname & Linux System Administrator's Guide