In most respects, computer speakers work in exactly the same way as other speakers. An oscillating signal flows down the speaker wire from the computer into the speakers. This signal looks like a sound signal in that it gets stronger and weaker thousands of times each second in a pattern that corresponds to the sound being played. Most computer speaker systems actually have two signals--one for the left speaker and one for the right.
The Voice Coil
The signal flows into the voice coil, which drives the speaker. A voice coil is a coil of wire right next to a magnet. When the signal gets stronger, it creates a magnetic field in the coil which pushes it away from the magnet. When it gets weaker, it decreases the magnetic field, and the coil slides back toward the magnet. The voice coil is attached to the speaker cone--a flexible cone of paper or cloth which moves in time with the coil--pushing the air every time it moves. All of these tiny pushes of air create the sound waves you hear coming from your speakers. The more air the cone pushes, the louder the sound waves are.
Computer Speaker Differences
The main difference between computer speakers and other sorts is that the computer speakers are more likely to have amplifiers and filters built right into them. These are called powered speakers. When you turn on your computer speaker, an amplifier increases the strength of the signal. The signal coming from your computer is not very loud, but with the amplifier on, some computer speakers can make a lot of noise. Computer speaker systems with subwoofers or tweeters included may also have filters built in. These filters split up the sound, sending high frequencies to the tweeters and low frequencies to the subs.