Ok, you need little education.
DVD-RW & DVD+RW can be erased, e.g. the disk is wiped with the laser to it's
"virgin" condition ready to be reburned. Usually there's a speficic option
within the burning software to do this but also usually there's a dialog
allowing you to first erase the disk before use if you put in a non-blank
disk when starting burn.
DVD-R & DVD+R cannot be erased, once a specific area of the disk is used
that's it. How-ever a disk can be partially used allowing one to "come back"
to the disk later and add more to the disk. If what's added conflicts with
what's already on the disk (e.g. an updated file with same name) the old
file is "forgotten" with a new table of contents written to the disk to
update the change. Notice the old area essentially ceases to exist but is
not reclaimed for future use. Eventually the disk will be entirely used up
even if it's just a one byte file repeatedly written to the disk.
To learn more do a little research on "sessions" and "finalizing" disks.
Also note this is different than "packet writing software" which treats a
disk as another hard drive, e.g. copy/paste etc. directly within explorer.
Note however using just -/+R (rather than RW) leads to same place.
