Open-back vs Closed-back
This question comes up often and it's actually very simply put. Open headphones will generally have a wider sound stage, giving the music more room to breath and usually more realistic presentation. It can be thought of as seeing a concert outdoors where the music has an air to it. This isn't always true, but it's one way to look at it. The downfall to open headphones is that they leak sound and let sound in. This makes them poor solutions for traveling or if you're sharing a dorm room.
The rear of the speakers on the open-back headphones is covered by an acoustically transparent mesh or fabric. This de-emphasizes low-end thump but delivers an overall "flatter," more even response that many audiophiles prefer.
Open-back usually results in more natural sound, but they leak sound in and out. This means you probably can’t use them in a library or a plane flight since the sound of your music will leak out and disturb the people around you. You also can’t use them on loud public places since noise from the outside will disturb your music. Therefore, even though open back would give you more natural reproduction, for these reasons their use is limited to mostly at home or in the office (if you have your own space that is).
Open-back headphones typically require more amplification power than closed back. For example, some open back headphones like Grados and Alessandros still fall into the semi-portable category, but they would benefit from an amplifier because of their open back design.
Closed-back designs feature solid, acoustically isolating earcups. They excel at blocking out external noise for a private listening experience, but also colour the sound (most notably, enhancing bass response). Closed-back headphones generally have a more narrow and direct sound presentation, similar to going to a club or an intimate venue. This isn't always true either, but it's one way to think of it.
Closed-back are not always inferior to open-back headphones. They are usually easier to drive, making them a good choice for portable set ups. They also tend to have punchier bass which is good with most mainstream modern music.
Circumaural vs Supra-aural
These can be open-back or closed-back headphones. They don't matter.
- Circumaural headphones (sometimes called full size headphones) have circular or ellipsoid earpads that encompass the ears. The defining characteristic of these is that they will completely cover your ear inside the pad. Because these headphones completely surround the ear, circumaural headphones can be designed to fully seal against the head to attenuate any intrusive external noise.
The pad of the headphone generally won't touch the ear, rather make a seal around the outside of the ear. These headphones tend to be large and are generally not ideal for portable use. Because of their size, circumaural headphones can be heavy. Ergonomic headband and earpad design is required to reduce discomfort resulting from weight.
- Supra-aural headphones have pads that press against the ears, rather than around them and covering your ears. Some users find the design to be uncomfortable due to the pressure applied on the ears. They were commonly bundled with personal stereos during the 1980s. This type of headphone generally tends to be smaller and lighter than circumaural headphones, resulting in less attenuation of outside noise.