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The elongated bumps that make up the track are each 0.5 µm wide, a minimum of 0.83 µm long and 125 nm high. They look something like this: 
You will often read about "pits" on a CD instead of bumps. They appear as pits on the aluminum side, but on the side the laser reads from, they are bumps.
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How optical discs are read
A CD is read by focusing a 780 nm wavelength- infrared level, semiconductor laser through the bottom of the polycarbonate layerThe common principle for optical discs is the reflection of laser in interaction with the polycarbonate data surface. The change in height between pits and lands results in a difference in the way the light is reflected. By measuring the intensity change with a photo-diode, the data can be read from the disc. 
Each string of 1s and 0s corresponds to an electrical signal (a voltage). The DAC (digital-to-analog converter) turns the numbers into voltages. The voltages change 44,000 times per second! The amplifier sends the voltages to the speakers where they turn into a series of sounds.