Basically, on the CD are etched a lot of little indentations, like square valleys. So each line of data spiraling out on the disc looks like a series of indents. Flat area, indent, flat area, etc.
The CD player bounces a red laser beam up onto the bottom of the disc, then waits for the light to return. If the beam hits a flat area, the player recognizes this (the time it takes to return) and registers this as a 0. If there's an indent, then it registers it as a 1. Reading all these millions of indents, the CD player constructs a huge code of binary computer language, interprets the code as instructions, and does what that code says.
DVDs and Blu-Rays work on the same principle, except Blu-Rays use a blue laser. Blue light has a thinner wave than red light does, so the indents on the Blu-Ray disc can be smaller, and therefore the manufacturers can pack more indents on the disc, which means more information
Source(s):
A science presentation my friend did in our 11th grade physics class, which was very informative.


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