Headphones - sometimes also called cans - seem simple. You plug them into a jack, put them on your head, and listen to music.
If only they were.
In reality, there are many varieties of headphones, and just like speakers, they're all slightly different. While they both fit on your noggin and make sound, the difference between a pair of Sure E3Cs and some Stax electrostatic headphones is huge.
Thankfully, there are a few things that can make deciding the right headphones easier. There are really only four types, and variations upon them: open-back, closed-back, in-ear (earbuds), and electrostatic. In addition, they all have several varying characteristics which much be taken into account.
The standard-issue pair of headphones is "Dynamic" - it has a magnet and a cone and a little coil, and works just like a tiny speaker. This single driver produces a full range of sound - hopefully, from 20hz to 20khz, at least in theory. However, much like any other speaker, different headphones have different frequency response - some have a far better range than others.
However,because they're so close to your ear, headphones don't need to make nearly as much sound, and use a tiny amount of power. As a result, a pair of headphones' impeadance is usually much higher than that of normal speakers - usually between sixty and six hundred ohms. Headphone impeadance and sensitivity are both important factors in deciding the right headphones for your application - a pair of 600 ohm headphones simply won't be as loud as a pair of far more sensitive 60 ohm headphones on the tiny voltage output of an iPod.
Of the four types of headphones, closed-back are generally the most common. They have a small chamber on the other side of the driver from your ear, and hence do a very good job of eliminating outside noise. Though small headphones do exist, the lack of a good seal around your ear generally results in much poorer sound quality, and as a result, most high-end headphones are very large with big cups that fit over your ear. Some popular examples of these include Beyerdynamic, AKG, and Sennheiser.
Another, somewhat less common, type are open-back headphones, which lack the outer chamber. While this can help eliminate back resonance and, as some claim, distortion, the lack of a rear chamber results in a lot more outside sound getting through. Popular examples of these are the Grado SR60's and SR80's, which have won a variety of accolades for high-quality sound at a low pricetag over the years.
Of course, a good pair of cans is an inherently bulky object. Earbuds work slightly differently, vibrating a small "plug" that is sealed in your ear. (As a rule of thumb, proper earbuds will work much better than pseudo-earbuds that only fit loosely in the ear.) Because they fit snugly in the ear, well-made earbuds can block more noise than any other kind of headphones. Excellent earbuds can be had from Etymotic Research, Sure, and several other manufacturers.
Finally, there are electrostatic headphones. With the exception of oddball electrostatic-hybrid headphones like the AKG-340s, these headphones require high-voltage sources - also called "energizers" - to create the electrostatic charge necessary for them to function. In addition, they're also highly inefficient, requiring far more power than any other type of headphones.
On the other hand, electrostatic headphones can have far better high-frequency response than any other type of headphones, and can dodge many problems with resonance or energy storage found in most dynamic headphones. While not portable, electrostatic headphones are regarded by their owners as far superior to most loudspeakers - then again, there are many others who disagree with them.