Many people enjoy listening to music on the computer, through headphones or on a stereo system. Few people take the time to realize how the music is being created and what role the speakers and the amplifiers play in that.
The stereo is the actual item that creates the sound through the vibration of the sound cone, but it has nothing to do with how loud the music comes out. When you turn the stereo sound up or down, that is the role of the amplifier. It takes the original signal and increases the amplitude so it sounds louder than the original input.
The speaker can produce sound without the amplifier, but the amplifier is useless without a speaker. Without the amplifier, the speaker would work, but it would only be able to produce the sound output at an equal volume to the input. The amplifier would be able to change the volume, but without the speaker to actually create the sound, nothing would be heard.
The amplifier and speaker are part of any electrical system used to create and change the volume. Musicians use speakers and amplifiers to create music from their instruments. Stereos, headphones and any other devices used to listen to music or any other sounds use the amplifier and speaker combination.