DVD-RW (Book F of the DVD Forum) is the rewritable version of DVD-R

Tech

It uses a phase-change recording layer, much like CD-RW,  groove recording with address info on land areas for synchronization at write time (land data is unnecessary during reading).

Media

Initial capacity will be 4.7 GB. DVD-RW media don't require a caddy or cartridge, but maybe some though some drives may take the additional precaution of requiring that the disc be in a caddy.

Where we could possibly read it?

Developed by Pioneer based on DVD-R, using the similar track pitch, mark length, and rotation control, DVD-RW will be playable in most DVD drives and players. But seems that some DVD-Video players, when fed a DVD-RW disc, "see" the lower reflectivity and "assume" that they should be trying to read a dual-layer disc. It is expected that will be fixed by firmware upgrade.

Price

Pioneer's coming drive estimated about $1600. The media (4.7GB) around $20.