Double Layer vs Dual Layer
The terms are Interchangeable - THEY ARE NOT !!
Double Layer - standardized by the DVD+RW
Alliance for the plus ("+") format and given the formal name "Double Layer."
Dual Layer - standardized by the DVD Forum for the dash format ("-"), which it dubbed "Dual Layer."
NOTE: as of now, drives are only available in write-once formats called DVD+R DL and DVD-R DL. However, DVD-RW DL and DVD-RW DL are right around the corner.