There is a fundamental difference between guitar cables and speaker cables in their internal design and use. For beginners, the differences may at first seem trivial, but the proper use of each cable is quite important.
Guitar cables require wire shielding to filter out radio frequencies, while speaker cables do not. Using speaker cables with a guitar will result in loud humming and possible interception of radio waves, making sound quality unacceptable.
Both guitar and speaker cables use two-conductor wire. Speaker wires carry voltage to the speakers, so a larger wire core size is necessary. Since guitars produce very low voltage, thinner wire core sizes are used.
The two speaker wires are of even size, as both the positive and negative are required to carry the same electrical voltage. Guitar cables use a single "hot" positive wire, and a wrapped or braided negative "ground" wire, which doubles as a shield. The hot and ground wires on guitar cables are different sizes.
Speaker and guitar cables are very similar in external appearance. Both use external insulation and 1/4 inch phone plugs, often making identification from the outside impossible. Although some cables are marked externally, many are not, or the markings may be difficult to read and decipher.
Since outside cable identification marks are sometimes difficult to read or non-existent, the wires can be identified by unscrewing the plug cap. If both wires are wrapped in insulation, they are speaker cables. If one wire is wrapped in insulation, and the second wire is not, it is a guitar cable.
Using speaker cables with a guitar will result in noise, but it poses no danger to the guitar, amplifier or cable. Using guitar wires for speaker connections will not only produce lower sound levels, but may also short out the cables, causing eventual amplifier or speaker damage. There is also the risk of fire, as guitar cable wires are not thick enough to handle higher voltages.