DVD-ROM stands for Digital Versatile Disk - Read Only Memory, they are different from other DVD formats because DVD-ROM discs can only be read and not written to.
A DVD differs from a CD as it can hold a lot more data. DVD-ROMs are often used for Films or TV shows as they can only be read and not written over.
The DVD-ROM has largely replaced the video cassette, being far more efficient and superior in most respects.
A lazer beam in the DVD player tracks the beam as the disc spins, while a special device reads the intensity of the reflection as it bounces off the pits and lands. The reflective variance gets translated to bits of data which form bytes. As a result, DVDs can vary in capacity.