/DOS is a subdirectory which is present on some Linux systems (such as muLinux) which also have a DOS or Windows OS installed. On these machines, /DOS contains the entire DOS filesystem, so that you can browse your DOS/Windows files from within Linux, which is useful if you tend to use your Microsoft OS more often than Linux.

Generally, your Linux kernel will not just make a copy or image of your DOS filesystem, but access the actual filesystem itself. As such, you'll need to be careful about using rm -rf / or else you'll lose both your Linux-managed and your Windows-managed data.

And before you ask, if you boot up your Linux kernel from a Windows folder (say C:\LINUX, as I do) then generally your Linux kernel will hide this folder so you can't access it whilst using Linux. Got that?