There are a few differences. Guitar leads are designed to carry very low current and are shielded. If you open up the connector you will see a braided wire (ie) the shield. Speaker cables arn't shielded (if you open up the connector you will just see 2 wires soldered to the connector) but are designed to take the high current that is transferred between the head and cab. For instance a 100W head will be looking at a current around the region of 3.6 amps. This is far too high for a guitar lead. If you use a guitar lead between your head and cab, the cable can overheat due to the high current and lead to shorting of the amp. Shorting of the amp can lead to bad damage to the output transformer and that can be a very costly fix.