Does writing to a DVD-RW erase all data that is currently on the disc?

It seems to be doing so.

What i want to do is to be able to  use the disc as a very large floppy,  where i can add to the disc and not destroy the current data.  

Am i using the right media? Disc's and drives? Or am i doing anything wrong?

More to add: How do i find  out what kind or brand of drive that i have?

Any help would be great