This lets us make smaller grooves on the disk, as well as smaller pits, and so we can encode more information on the disk. In addition, both sides are used, basically by gluing two DVD disks together. CDs contain about 70 minutes of sound information. DVDs can hold much more than 2 hours of full video information. It's very impressive technology.
Modulation is the changing of a signal from one form to another.
You modulate your voice, adjusting how loud you talk, so people can hear you in different surroundings. In music, changing from one key to another is also called modulation. In computers, the modem modulates a computer signal so it can travel over phone lines because computer signals and phone signals are of different kinds. Computers send discrete bits of information, ones and zeroes. This kind of information is called digital.