what is a double layer DVD burner?

Movie DVDs you buy in the store can contain up to 8.5 gigabytes of data. This is because they are pressed (as opposed to burned) dual-layer DVDs (also called DVD-9). The recordable DVD+-Rs that have been on the market for a while a single-layer DVD-5s and can store up to 4.7 gigabytes. This disrepency is why programs that copy movie DVDs need to compress the DVD content after unencrypting it before they can be burned to a normal DVD. Recently new DVD burners and media were introduced that allowed people to burn DVD-9 DVDs reasonably affordably.So a double-layer DVD burner is one that is capable of burning 8.5 gigabyte DVD-9 format DVDs.http://en.wikipedia.org/wiki/Dvd#Technical_information