Your USB device will be /dev/sd[b,c,d,...] (depending on how many other drives you have). Some systems (like those with Gnome) automatically mount your USB drive to something like /media/DEVICENAME (depending on the usb drive's name, id number). If you need to mount it manually, you can do:
sudo mount /dev/sdb /mnt
(where sdb is the usb drive). After that, you can do:
mv /mnt/somedir $HOME/Desktop
Keep in mind, mv will put the file/dir in the new location and delete it from the old location. cp will make a copy.