The actual cable, the wire itself, is what is different about speaker versus instrument cable.
Instrument cable is shielded. There are two conductors, an insulated central core wire, usually stranded, with braided-strand wire wrapped around it as a shield against noise.
Speaker cable is just like lamp cord. Two conductor, stranded wire, running parallel, not one wrapped around the other.
If you use speaker cable to connect a guitar to an amp, there will a loud buzz. It will sound bad.
If you connect a speaker to an amp with an instrument cable, it will sound OK until you turn the amp up to play loud, then the amp will blow up.
The plugs on either type of cable are EXACTLY the same as any other "quarter-inch phone plug".
They are called "phone plugs" because that's what was used on old-time telephone switchboards to route phone calls.
The PLUG is the thing at the end of a cable. It's "male".
The JACK is what the PLUG goes into. It's "female".
Don't laugh - idiots interchange the terms willy-nilly. Makes you wonder how they ever manage to reproduce.