"explain the concept to me"
Linux uses one large file tree. This tree spreads across all devices which are capable of supporting file systems. You can insert new devices into the file tree using the mount command. You can take devices out of the file tree using the umount command.
For example: I have an unused partition on /dev/hda4. If I want to use this partition to store /home while I rearrange the other partitions I could do the following:
mount /dev/hda4 /storehome
cp /home/* /storehome -R
and /home is now duplicated on /storehome.
Or if I want to insert a CD-ROM at /home/user/software I put the CD in the drive and:
mount -t is09660 /dev/cdrom /home/user/software
and the files on the CD appear in the file tree at /home/user/software.
When I am through with the CD:
Be prepared. Create a LifeBoat CD.