DVD stands for Digital Versatile Disk or Digital Video Disk, depending upon who you ask ROM stands for Read Only Memory. A DVD-ROM drive is able to play video DVDs, data DVDs, data CDs and audio CDs. A DVD-RW is also able to write to a recordable DVD disk.
Standard DVD disks have a capacity of 4.7 gigabytes or nearly three hours of video recording. A CD has a capacity of about 650 megabytes, less than 30 minutes of video. DVDs can be double sided and are available with dual media layers increasing the total capacity to over 13 gigabytes.
DVD-ROM drives are slowly replacing CD-ROM drives because they are nearly the same price but you get the added capability to play DVDs. However DVD-RW drives are roughly six times more than CD-RW drives and the media is 20 times more expensive. Because of their capacity, DVDs will undoubtedly replace CDs as the media of choice as the prices decline.
DVD players in PCs are of limited use because most people don't want to watch movies on their PC monitor. The exception is for laptops, they make great entertainment centers while traveling; although battery life becomes an issue.