Often used as a specification (measured in Ohms) for headphones and earphones, impedance describes how much resistance the headphone gives to the signal from the audio source. So any headphones with impedance between 20-40 Ohms is a reasonable choice for a casual music listener, and 64 Ohms and above for an audiophiliac, depending on the source.
Generally speaking, lower the headphones' electrical impedance, easier it is to get a higher volume and battery-drainage of your portable device will also be less. That being said, other factors can still limit loudness potential. Then again, it doesn't mean that higher impedance headphones are a letdown when it comes to volume. These will reach high volumes but will do so using high levels of power.
It is veryessential to match the power supply of the audio source with the impedance of your headphone. To avoid any kind of “blow outs”, low impedance headphones can’t be used with powerful amplifiers due to their low threshold limits. Higher impedance headphones displace all hissing sounds produced by high quality headphones. But the downside, is that higher impedance demands higher power, and thus not necessarily work very well with your iPod or Mp3 player, which is why a dedicated headphone amplifier is required to supply enough power to reach high enough listenable volume level.