1.) What’s the difference between a speaker cable and an instrument or guitar cable? 

Instrument Cables have thinner gague wiring, a shielded ground, and are acceptable to use on guitars, keyboards, signal processing, etc.

Speaker Cables have thicker gague wiring, an unshielded ground, and are used to send powered signal through.

2.) What are similarities between a speaker cable and an instrument or guitar cable?

Both typically have 1/4″ ends, are two wires running through a single cable in a hot/ground configuration (tip/sleeve ends). They both have the same look to the ends and sleeve of the cables (and often times the only indication that one is a speaker cable or an instrument cable is written on the cable)

3.) What happens if I use a guitar cable instead of a speaker cable when I should be using a speaker cable?
Because the wiring is so much thinner on a guitar cable, the power can melt the cable, shorting it out, causing damage to your system.

4.) What happens if I use a speaker cable instead of a guitar cable when I should be using a guitar cable?
Being unshielded, the speaker cable can buzz and add noise to your signal.