A very simplified definition, though not technically correct, would be:
Resistance: opposition to current that is constant no matter what the frequency (60 hz, 2kHz, 1MHz, etc.)
Impedance: opposition to current that varies as frequency varies.
A resistor gives a constant resistance at all frequencies.
A capacitor gives more "resistance" as freqency decreases.
An inductor gives more "resistance" as frequency increases.
The audio signal has a varying frequency, from around 10Hz to well over 20kHz. So it becomes an issue with cable designs to account for this in order to minimize the effect of cable resistance. For speaker cables, a lower overall impedance improves the amplifers' damping factor which minimizes the distortion by spurious signal oscillation from speaker-to-amp-to-speaker. As far as interconnects go, Sean covers it very well.
Resistance: opposition to current that is constant no matter what the frequency (60 hz, 2kHz, 1MHz, etc.)
Impedance: opposition to current that varies as frequency varies.
A resistor gives a constant resistance at all frequencies.
A capacitor gives more "resistance" as freqency decreases.
An inductor gives more "resistance" as frequency increases.
The audio signal has a varying frequency, from around 10Hz to well over 20kHz. So it becomes an issue with cable designs to account for this in order to minimize the effect of cable resistance. For speaker cables, a lower overall impedance improves the amplifers' damping factor which minimizes the distortion by spurious signal oscillation from speaker-to-amp-to-speaker. As far as interconnects go, Sean covers it very well.