The terms tweeter, woofer, and subwoofer refer to the range of frequencies the speaker is designed to reproduce.
Tweeter - high frequencies (treble)
Woofer - low frequencies (bass). The woofer in a non-subwoofer speaker reproduces bass but not as low bass as a true subwoofer.
There are many types of speakers that can be used for HT, whether they are booshelves, floor standing (sometimes called towers), and very large towers. All of them will have both a woofer and a tweeter with some designs having more than one woofer or tweeter.
The subwoofer is special in that it is designed to reproduce the very low frequencies that all but the largest and most expensive speakers cannot produce very well.
