User partition on internal memory: /mnt/free
CF card: /mnt/cf/
SD/MMC card: /mnt/card
USB-stick: /mnt/usb
For the unix/linux specific file hierarchy see:
http://www.pathname.com/fhs/
http://en.citizendium.org/wiki/Unix_directory_structure
or
http://en.wikipedia.org/wiki/Filesys...archy_Standard
And just a basic rule of thumb: don't touch anything unless you know exactly what it is.
hth