A phase switch adjusts the timing of your powered subwoofer very slightly in order to help its woofer and the woofers in your main speakers move in and out at the same time (or "in phase"). Depending on its placement in your room, your sub may sound better with the switch engaged, or not.
Speaker-level inputs let you hook up your powered sub by using the speaker connections from your receiver or amp, eliminating the need to run a separate RCA patch cable.
If you're using speaker-level connections to hook up your powered subwoofer, speaker-level outputs let you easily send amplified signals on to your main speakers.
Woofer Size (inches)
The diameter of the woofer.
Some computer speakers feature a separate subwoofer for bigger, more powerful bass.
Amplifier Power (RMS)
The subwoofer amplifier's continuous (RMS) output in watts. The greater the RMS output, the cleaner and louder the sub will play.