Just like you have tenors, basses, sopranos and altos in a choir, different sizes of speakers handle different ranges better. This is why a speaker system generally has more than one actual speaker built into it.
To achieve a quality sound system, especially for use by singers, bands and DJs, you will generally have a setup that features speakers and subwoofers, as well as tweeters, mid-ranges and woofers, because they each handle a different range of notes and sounds and when used together, the entire range of sound is covered.
What is a Subwoofer
A "sub" or subwoofer is actually a woofer, or a loudspeaker, and its purpose is reproduction of low-pitched audio frequencies. This is also referred to as bass in common language.
Subwoofers Form and Design
As for a subwoofer's structure, it is usually a wooden or plastic loudspeaker enclosure that is fitted with one or more woofers. The manner in which the woofers are placed creates room for the birth of several subwoofer variants. Subwoofer variants are distinguished depending upon the efficiency, size, cost, power handling and distortion characteristics that they display. The designs created thus include bass reflex, horn-loaded, bandpass and infinite baffle subwoofers.
The frequency range that a subwoofer delivers depends on its use. So a subwoofer for home-use would have a frequency that would typically range from 20-200Hz; while that used for professional sound would be below 100 Hz. There are subwoofers with frequency below 80 Hz as well.
Types of Subwoofers
Active subwoofers and passive subwoofers are the two options that you would be required to choose from in case you wish to buy them. The active subwoofer boasts a built in amplifier, while the latter needs an external amplifier, and comprises of just the subwoofer driver and enclosure.
What is a Speaker
A speaker on the other hand, is nothing but a loudspeaker. A speaker refers to an electro-acoustic transducer. It converts an electrical signal into sound. The inherent working principle for a loudspeaker (or speaker) thus is that the variations of an electric signal cause the device to move in sync with it, and propagate sound waves through the medium of either air or water.
It is the speakers that cause the maximum distortion and audible differences, thus playing a significant role in comparing audio systems.
The difference between subwoofers and speakers basically pertains to their frequency range then. While subwoofers are used for the lowest part of the audio spectrum and are perfect for bass sounds, there is no such restriction with speakers and they deliver high frequencies, like mid-and treble range, to perfection.