Adding two (or more) waves to create one resultant wave is called superposition. The images below shows the superposition of two different sine waves (blue and green) to form a resultant (red) wave.
This process occurs whenever two sounds are heard simultaneously from the same source – such as a snare drum and electric guitar heard simultaneously on a recording. The snare drum sound wave and the electric guitar sound wave have been combined during the recording process to create a single sound wave which is stored (and played back) by your iPod, CD player etc.
Note that it is possible for two waves to cancel each other out if they are of equal amplitude and opposite phase.