DVD-ROM employs same principle as a CD-ROM for reading and writing. However a smaller wavelength laser beam is used. Total capacity of DVD-ROM is 8.5GB. In double-sided DVD-ROM two these disks are stuck back to back that allows recording on both sides. This requires disk to be reversed to read reverse side. With both side recording and with each side storing 8.5GB total capacity is 17GB. 

In both DVD-ROMs and CD-ROMs the density of data stored is constant throughout spiral track. In order to attain a constant readout rate the disk should rotate faster near centre and slower at outer tracks to maintain a constant linear velocity (CLV) between head and CD-ROM/DVD-ROM platter. So CLV disks are rotated at variable speed. Compare it with mechanism of constant angular velocity (CAV) in which disk is rotated at a constant speed.  So in CAV the density of information storage on outside sectors is low.