DVD-RAM is just another format to write on DVD's.
We have a bunch of them (DVD-R, DVD+R, DVD-RAM etc.)
The thing is that there aren't that many vendors that support the DVD-RAM format so if you buy a DVD-RAM recorder, and burn a DVD... go to your living room, if your setop DVD player doesn't read DVD-RAM... you got a problem. You can only access that disk at your puter. Thats why nowadays, most DVD Burners can use both popular writing formats + and - (and read -RAM)