Patch and instrument cables are shielded audio cables that rout the signal from one source to another, beginning with your instrument and ending with the power amp. Shielding is when the ground wire is braided around the other wires in the cable to guard against hum. The copper wires within these cables are usually 20 gauge, which is much smaller than speaker wires. By using good patch cables and keeping them as short as possible you can prevent signal loss. Quality is especially important for the cable from your instrument, as this is conducting the weakest signal.
Speaker cables connect the amp to the speaker.
They are not shielded and they use large diameter wire, typically 12 gauge. These cables must conduct a large amount of power to the speaker without losing information or altering the signal in any way. All wire is not created equal and all copper is not the same. Acoustic instruments have many subtle nuances that can be lost by using inferior cables. You don't have to use the most expensive esoteric cables but good, solid quality is necessary to get the best from your amplifier and your acoustic instrument.