2.0 = stereo speakers, 2 of 'em, Left and Right. Bass response is limited by speaker size, power and design.
2.1 = Stereo plus a subwoofer, bass frequencies are handed to the single subwoofer speaker which is designed specifically for low bass reproduction (under 100 Hz), allowing the Left and Right speakers to be designed for the midrange and treble. Best purchased as a matched set. Single subwoofer because low bass frequencies are theoretically omnidirectional, you don't need to separate left and right.
4.1 - 7.1 = surround-sound implementations -- various combinations of Back, centre and side speakers in addition to the Left and Right (front) speakers and the subwoofer. Again best purchased as a set. These usually require a separate audio input for each speaker (other than the sub-woofer, which is optional). Some of them will simulate surround sound if given only the L and R inputs. Only a few models have a DTS or Dolby decoder built in which can decode a multichannel signal.
NOTE: there is currently little surround content in stereo CDs and most music content you will play on your Mac. In order to get anything more than simulated surround, you have to have multichannel source material and a playback software that handles it, plus the correct hardware. Currently only some DVD movies and some games are encoded for multichannel, and some rare Super-CD or DVD-Audio formats. Don't know that the Mac can handle those CDs playback anyway.
The only true surround sound you will get from a Mac without additional hardware is through the optical audio out port on PowerMac G5s and the 17" Powerbook G4. Those require a speaker set with a digital audio input and the appropriate decoder hardware. $250-$300 and up.
You can get a 7.1 USB audio interface (Sonica Theatre from M-Audio) and hook up to conventional surround speaker systems. But remember the caveat about sound sources.
Dolby and DTS are rival encoding methods for digitally wrapping multichannel sound into an audio file. Using them requires decoding by the same method, usually in hardware built into a powered speaker set or a receiver. If it doesn't say that it has Dolby Surround, Dolby Pro or DTS, than it doesn't have the necessary decoder.
http://en.wikipedia.org/wiki/Surround_sound
http://en.wikipedia.org/wiki/Dolby_Pro_Logic
http://en.wikipedia.org/wiki/Digital_Theatre_System