A DVD-9 is a single sided, dual layered DVD. It can hold 8.5 gigabytes of data, which comes down to approximately 241 minutes of film.
The discs are 12 cm (4.7 inches) wide, and 1.2 mm thick.

Because a stronger laser is used, which can be focussed better, it is possible to access a second layer, behind the first layer. This is accomplished by placing the data on the upper layer a bit further apart then normally, and then the laser can read between it to access the second layer.

The advantage of using a DVD-9 to a DVD-10 is that the disc doesn't need to be flipped. A minor detail is that it takes about 1 or 2 seconds to switch the laser from the upper layer to the lower layer, but this can go unnoticed if this is between scenes.