Design:
Open-Air & Closed-Back Headphones:
 Open-air and closed-type headphones refer to the back of the ear-cup being open or closed.  Open-air headphones are meant to let you hear the environment around you by allowing sound to pass through the ear-cups freely. Closed-back headphones are sealed, blocking out background sounds. Closed type earphones will out perform open-air models on basic performance because the music is more focused without having to contend with background noises. Open-air headphones normally have less distortion then closed-type headphones since the sound isn't restricted. The atmosphere the two create are completely different, on the one hand you are blocking out ever thing but your music, and on the other hand you can hear-through your music.

Active and Passive noise canceling:
 Passive noise canceling headphones rely on noise suppression techniques, for example: a closed-back, circumaural headphone is naturally a passive noise canceling headphone.  How well it works comes down to the quality of the ear-cup system. Passive noise canceling earphones are normally good at blocking out the high frequencies, but blocking out the lower frequencies is where the active noise canceling headphones come in.  Active noise canceling headphones are the same are passive noise canceling headphones but use microphone placed in the ear-cup to pick up incoming sound waves and sends them through a electronic circuity to generate a anti-noise sound wave to send back out, canceling the incoming sound waves.

Headphone Form-factor:

Circumaural headphones:
 Circumaural headphones have ear-cups that go over-the-ear. Normally circumaural headphones are larger and are used for home and pro applications. Although the ear-cups cover and seal the ears, circumaural headphones come in open-air and closed-type designs.

Supra-aural headphones:
 Supra-aural headphones are meant to sit on top of the ear. Normally supra-aural headphones are smaller and used for portable application or open-air headsets. By design the supra-aural headphones are naturally open-air since they do not cup the ear. However, some models will still come in a closed-type (closed-back) design.

Ear-Buds:
 A ear-bud is meant to sit just outside of the ear-canal, while having some type of ear cushion softly filling in the remaining space. Most ear-buds come stock with portable players and are not meant to seal off the ear completely, making most ear-buds a semi-open air design.

Inner-ear / Canal-phones:
 Inner-ear and canal earphones are also known as In-ear Monitors or IEM's. They are a type of ear-bud that is meant to fit directly into the ear canal, sealing off background sound and improving performance. Most inner-ear earphones are a closed-type, passive noise isolating earphone.


Features:

Volume Control:
 Although this does sound like a handy feature, selling earphones has really turned me against volume controls. I would say at least 7 out of 10 customers are unhappy with volume control models, but are happy with the same model minus the volume control. The reason is once you put another volume control into play there are many areas of distortion. It comes down to having both volumes in the right places. For example if your player is at max volume and the ear-buds are and half volume, you are going to have distortion. Now if the player was only at 3/4 volume with the earphones at half, you would be fine. Anyways, in 2007 most mid and upper end earphones stopped including a volume control model, I would say simple because most are unhappy with the v/c model. The talk is of mute buttons but none of the big players are doing it yet.

Magnet Type:
 Magnet type can be a very important feature to look for. When buying a pair of portable headphones, Neodymium magnets are the best way to go. Although all types have their unique benefits neodymium magnets pack more magnetic energy per ounce than conventional Ferrite, Samarium Cobalt or Aluminum magnets, producing higher sensitivity and ample sound output from less power.

Neck-Strap Vs. "Y" Cord:
 We've all seen the "Y" cord style headphones with maybe 98% of all headphones having a "Y" type cord it's hard not to. However, in portable earphones Neck-Strap cords are becoming much more popular.  The neck-strap cord is not your normal cord, after the cord splits to go to each earphone the cord lengths are different. The left cord is between 5-7 inches long and the right earphone cord is between 20-21 inches long, really a shocking difference in length. I remember once reading a review done on the neck-strap cord and the reviewer said that he hated it until he tried going back to the "Y" cord version. At the time JVC had just released it's fx55 series with the Z-cord (neck-strap), so I gave them a try. My first impression was how dumb can some companies get, it really seemed like a joke. However, a few months later when the new panasonic's came out, I bought the "Y" cord version instead of the neck-strap. Like the reviewer, I just can not go back to the "Y" cord now.

Connection Quality: Cord/Plug
 Having the right power is great but getting the power to the drivers is another trip. Since high quality metals conduct power better the lower quality metals the quality of the plug and cord matter. Most stock earphones come with a standard copper cord and a nickel plated plug. Two great features are Oxygen Free Copper Cords and Gold-plated plugs, both of these features will improve connection quality, Which makes for a better power delivery system and improves overall sound clarity.

Plug sizes:
 The two most common headphone plug sizes are 6.33mm (1/4in.) and 3.5mm (1/8in.). The 3.5mm was introduced in 1979 by Sony(r) and became the most widely used connection type, everything from PC to mp3 players. The older standard 1/4in. connector is still used in professional audio, however most pro line headphones are a 3.5mm jack, but will include a 1/4 adapter. Adapters are sold to flip 1/4 and 3.5mm either way and are highly common. As for home and mobile phones, most headset ready phones use a standard 2.5mm plug. However, many mobile phones are not a standard plug size and require a adapter normally purchased from the mobile phone manufacture.