What's the difference between a dual layer DVD and a single layer DVD?

Single Layer
These are the standard DVD discs that most people have been using for years.

Their code name is DVD5 and they hold around 4.7GB (It would have been 5GB but the design specs set aside around 300MB for indexing of the disc hence 4.7GB)

These discs are very common and very cheap to manufacture, and with the use of a program like DVD Shrink can be used to backup most movies.




Dual Layer
As the name would suggest there are actually 2 layers to these discs. Simply put the laser focuses on one layer and reads 4.4GB on one layer, and then it refocuses on the layer above it and burns another 4.4GB of data.

With these discs you get nearly twice the storage capacity, but they only work with certain burners and they tend to burn at a slower speed.

The major advantage of Dual layer discs is that you can backup most film DVD's without having to compress them.





How to tell the difference
The simplest way to tell the difference is from the logo's. For Dual Layer DVD's you are looking for DVD+R DL or DVD-R DL.

The other obvious way is the capacity of the discs, but you can't tell the capacity through Windows XP or earlier, and will have to use a 3rd party program like Nero or Roxio
or to use a program specifically designed for DVD identification like DVD Identifier or CD Speed

Link:
www.medea.co.uk/medeasupport/index.php?action=kb&print=76