The frequency response is the sound range (frequency) which a driver can effectively reproduce. It should be noted that the human ear is only able to pick up between 20-20,000Hz so anyone that can hear above that is either a very seasoned audiophile or is lying to you. Frequencies below 20Hz are more often felt then heard, accounting for the "thump" of a good subwoofer.
Impedance is a measure of headphone load on an amplifier, which is measured in ohms. Most headphones on the market have impedances of less then 100 Ohms. Professional headphones rated at 200 Ohms and above minimize loading effects on distribution amplifiers, which may drive a bank of headphones. The higher the impedance, the more power required to drive the headphones. For this reason, many enthusiasts also purchase a headphone amplifier for better volume levels.
THD is how accurately a headphone reproduces sound and is given a percentage of the signal distorted. Tests have shown humans can distinguish about 1%, so the THD of the 600's is extremely impressive.
The amount of pressure placed on the head measured in Newtons. The maximum pressure allowed is 5N; at 2.5N, the 600's ensure maximum comfort.
If headphones are worn for any extended period of time, you are going to want them to be comfortable. Plastic headbands and short cords may be sufficient for a new Walkman purchase, but anyone really concerned with comfort knows that a plastic headband is going to hurt after an hour. This is where Sennheiser has stepped in with the comfort factor.
One of the amenities of the 600's is the plush ear cups. Covered in velvet, they fully envelop your ear, putting pressure on the head instead of directly on the ears. While seemingly insignificant, it is surprising how long the 600's can be worn without any discomfort.
The plastic headband is still there, but in order to facilitate even better comfort, foam padding has been added in between the plastic and your head, making them rest lighter.