Double sided media has a single layer of recordable face on each side, with 4.7GB of storage capacity on each side making up to 9.4GB total storage capacity. When play back, changing side is required when one side finishes playing. Recording movie or single file larger than 4.7GB requires special software that splits the video or file into two parts.

Double layer media has two layers of recordable face on a single side, with 8.4GB of storage capacity. Recording to double layer media requires burner or recorders that supports double layer recording usually it is indicated by a DL sign.

Souce

Here you go. Well, I don't think there got sell double sided dvd nowadays. Why? It should cost more than the normal single layer dvd. In addition, you cannot burn more than 4.3Gb in that double sided dvd. So, how on earth people want to buy these kind of dvd? 

As for dual-layer (DL), it allows to burn up to 8Gb+. Most pc games nowadays are DL dvd cos' the setup size is more than 4Gb. 

Actually, they do sell those double sided DVDs, but its damn rare lar. The one (and only one) that I got is actually a rewritable DVD-RAM disk

A decent dual-layer DVD goes for around RM18 per piece. You don't need any special application to burn the DVD. I made a copy of Leopard using Disk Utility.