The document here will give you a good overview for mount points (filesystems).
Note that "mount point" and "device" are separate things when talking about filesystems.
mount point = The directory you mount the device on and from which you access the files in the filesystem. When the filesystem is NOT mounted this directory exists but is mostly empty (it should have only the . and .. hard links in it to pwd and parent directory). The one filesystem all UNIX/Linux systems will always have is "/" (root). There may or may not be others.
device = The partition, logical volume or media (e.g. usb or DVD) you mounted on the mount point. Examples would be /dev/sda2 for the second partition of your first hard drive, /dev/VolGroup03/lvol4 for the 4th logical volume of the 3rd volume group).
When you run df -hP you see the device in the left column and the mount point in the last column.
Note also that there are pseudo filesystems that are created at each boot (e.g. /proc, /sys on Linux with kernel 2.6 and greater).