6.2 Basic Concept

The basic concept behind DVD-R and DVD-RW are now well-known. The most fundamental concept is that these discs are compatible with read-only DVD discs. This is exactly equivalent to the relationship between CD-ROM discs, and CD-R and CD-RW discs. The basic discs are high-volume pressed versions, with compatible recordable media also playing a major role. Therefore, the 4.7 GB DVD-R disc is specified such that after recording, its signal characteristics and format of the recorded data are the same as those defined by the DVD-ROM specification. The same is true of DVD-RW; with the exception that the reflectivity of DVD-RW discs are the same as that of dual-layer DVD-ROM discs, DVD-RW discs are like DVD-R discs with the addition of rewrite capability.
Since all these specifications are defined based on this fundamental concept, DVD-R and DVD-RW discs can be easily played in players designed for read-only DVDs and in DVD-ROM drives.

Figure 2 Basic Concept

Source: http://www.pioneer.co.jp/crdl/tech/index-e.html