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.

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.