/dev is for device files, ie special files that let you communicate directly with a physical piece of hardware
/etc is for configuration files, ie the comments tell how to setup a device or application
/var is for variable files, ie files who's content is regularly changing, the primary example is logs, but there are a lot of other files that land in /var (some are moving to /run, though)
Pretty outdated. /run/media is used by udisksctl to mount stuff that the user actually has access to.