So there are 2 things you need to consider when you match a speaker head and an amplifier. Power handling (How many Watts it can handle) and impedance.

Wattage is pretty easy and most people understand it, but impedance is a little tricky. I like to think of impedance of like being the gears in your car, the lower the impedance (say 4ohms) is like running your car in first gear, and a high impedance (like 16ohm) is like running in 4th. So you might think why don't I just get a 2 ohm speaker and get the most out of my amplifier. Well what happens is that your amp will run so hard it will blow up? (Not great for your gig or your wallet)

Most amps will have minimum impedance written on them, so never go lower than this value. If you have a Valve / Tube amp it’s often 16 ohms or can be adjusted - just make sure it’s matched.

What About If I have 2 speakers - Can I get double the volume by running them both?

If you have 2 speakers in parallel (which is the normal way they would be run) then the impedance halves, if you have 4 in parallel then it halves again. There is a little formula to work this out, but I think it’s safer to only ever run 1 pair of speakers per amplifier (If you have a mono amp). Due to some funny laws of electronics and physics you don't get double the sound! It will sound a little fuller and can sound good but it’s only ever marginally louder in reality. If you’re running a PA, You’re often best to have only 1 speaker per frequency band so you don't get any sound cancellation.

Speaker Cables and Guitar Cables look the same why are they different?

Speaker Cables and Instrument cables have 2 very different designs.

An instrument cable is designed so that the central conductor (which has all your signal running down it) is protected or isolated by the shield (or ground wire) for those of you that want a more through explanation look up "Faraday Cage" in Wikipedia for a long explanation. So if ever you have a broken guitar cable you will see a single conductor in the middle and a thin earth shield wrapped all the way around it (Ideally with 100% coverage).

Now if you use this cable for your speakers, the wire is way too thin, and will easily burn out or short out (which can very quickly damage your amplifier)

A Speaker cable has 2 thick wires and no shield, the thicker the wire the better (It will have less resistance or amp loss, and won't get hot when you run a louder signal down it). If you happen use a speaker cable for your guitar (between the guitar and your amp), you will get mountains of noise (as it doesn't have any shielding)