2.1 speakers have an additional subwoofer channel (3 speakers in total). Typically source material isn't in 2.1 format, it's in 2.0 (or plain old stereo). The low frequencies are sent to the subwoofer by a filter or sound processor in an amplifier.
On sale
Taga Harmony inTONE 2.1 Mini Speaker System - Jamsticks
On sale
Taga Harmony inTONE 2.1 Audio System - TV Speakers - Jamsticks