Yes, there's a significant difference between an instrument cable and a speaker cable, primarily in their construction and intended use. Instrument cables are designed to carry low-level signals from musical instruments, like electric guitars or keyboards, to an amplifier. They are typically shielded to protect these delicate signals from external noise and interference.
On the other hand, speaker cables are meant to carry high-power amplified signals from an amplifier to speakers. They have a thicker gauge and are not shielded, as they need to handle a much higher current without significant signal loss. Using an instrument cable in place of a speaker cable could result in poor sound quality and potentially damage your equipment due to the mismatch in design and capabilities.