DVDs are great, but they have one limitation; how much data they hold. Not that long ago, the 4.7 Gbytes storage capability of a typical DVD seemed like a lot. With the influx of higher quality digital video, however, we need lots of data storage capability to hold all the video and audio information. A regular DVD cannot even hold a two hour movie of improved quality digital video. That's why the movies you purchase or rent are double layer (or dual layer) discs, and many game and application discs are as well.
 

Like the name implies, dual layer DVDs and double layer DVDs have digital information on two layers. Dual layer and double layer DVDs are essentially the same thing, and frequently the terms are used interchangeably. However, more accurately, the term dual layer is associated with DVD -R DL blank DVDs while double layer is used for the more common DVD +R DL blank DVDs. To simplify things, we will refer to them as double layer DVDs, but the information applies to both double layer and dual layer DVDs.

Double layer DVDs have two layers of data embedded on the disk, doubling the amount of data it can hold. Virtually every DVD drive or player can play double layer DVDs.

Now with double layer (DL) blank DVD media and DL drives you can burn DVDs that hold twice the data (8.5 Gbytes), including about four hours
of digital quality video. These DVDs will play on any drive or player. Twice the data means using half the discs.

Better video and more data on fewer disks. Double layer DVD capability is win-win.