A: At its most basic, a speaker cable is just a length of two-conductor wire (though there are, of course, variations and improvements on that theme).
An instrument cable, while also two-conductor, needs to be shielded, with a metal foil or wire mesh screen to prevent the pickup of RF, EMI, hum, and other menaces. Because instruments put out very low-level signals that are amplified immensely, any noise picked up by the cable will be commensurately amplified. A speaker cable, on the other hand is carrying signals that have already been amplified, so any noise they pick up will be inconsequential.
You could use a speaker cable to connect a guitar to an amp, but you will be very unhappy with the noisy results. Use the proper cable for best results.
You should NEVER, however, use a guitar cable to connect an amp to speakers — the lightweight conductors in the guitar cable are not intended to handle high-level speaker signals, and damage to the cable, amp, or speakers may result.