A DVD-W is a DVD of which can be re-written, with the previous set of data wiped, and fully replaced with a new set. These also have a storage capacity of 4.7GB.
This format is similar to a CD-RW, which can also have data removed or added to the disc.
The medium is not damaged in the process of writing data onto the disc numerous times, which proves as a perk for re-writable discs. Though the price differentiation between DVD-W's and DVD-R's is significant, this due to the sole ability to re-write data.
The function of a rewritable disc is slightly different to a normal disc, in regards to;
The dye layer upon the disc can be changed back and forth between opaque and transparent. This changing of the layers properties is what firstly allows data to be stored on the disc; though with the ability to constantly revert between the two, also allows the user to re-write data upon the disc.