Explanation Required Please: Coaxial vs. Optical vs. HDMI audio

[JJ]

My current sound system set up is linked to my BluRay player via a Coaxial Digital Cable - now Ive heard lots of explanations from the good people at Best Buy, but no concise explanation - what kind of sound am I getting from the Coaxial? 2.0? 5.1? Something Else?

Also, as far as the Optical S/PDIF or TOSLINK or w/e its called, is that a better alternative?

And last but not least, the only way to get HD sound is to hook up the player to a different receiver, like the Onkyo or Pioneer Elites, correct? But this would be a far more expensive investment than just a cable.