Question: What makes DVD-R write once and DVD Ram, RW and +RW rewritable?

Answer: DVD-R utilizes a dye similar to CD-R for recording. Once this dye is altered (burned/written) it cannot be changed (rewritten). The re-writable formats: DVD Ram, DVD RW, DVD+RW use a metal alloy material called phase-change, similar to what is used in CD-RW, to allow re-recording.