IMPEDANCE

Impedance is the resistance to a flow of alternating current. Represented by the symbol Z, it is measured in units called ohms (Ω).

Speakers are either low-Z (8 Ω) or high-Z (100V, 70V, or 25V). Both have a specific job to perform.

LOW IMPEDANCE

Low impedance is in a range of approximately 2 to 16 Ω (often referred to as 8Ω). Low Impedance is ideal for high performance applications, high powered speakers and short cable runs.

Low impedance sound systems are often found in night clubs, restaurants and patios, Houses of Worship, or your home or car stereo.

HIGH IMPEDANCE

High impedance is referred to as 25V, 70V, or 100V (often referred to as 70V). High impedance is ideal for longer cable runs, with more speakers per line. Speakers are low powered with adjustable power taps. It has high impedance outputs with fewer amplifiers required.

High impedance sound systems are ideal for public address, in corporate, commercial, industrial, retail or educational applications.

 ... our 70V basics webinar

While this is a fairly good explanation of application, the details are technically not really true.  

25V/70V/100V is not a measure of impedance but a description of what would properly be called a level of “constant voltage” amplifier output, and actually the true impedance (which is measured in Ohms, not volts) can vary considerably in constant voltage systems, depending on the actual physics of the system’s components and the real amount of resistance introduced by them.  

The real physics involved can be fairly complicated to explain thoroughly, unfortunately, which is why the descriptions tend to get simplified to 8-ohm vs 70-volt for ease of communication.  The true measure of impedance still comes in to play in constant voltage system designs but is expressed in terms of total speaker load wattage versus rated amplifier output, which makes them easier to design, as the math involved is much simpler than in a low-Z system (CV speakers have onboard transformers that convert the applied power to be appropriate for the driver). At the installation/troubleshooting stage it can be extremely important, hence an impedance meter should be used in CV installations to ensure there are no faults, etc.