[Closed-back Sennheiser headphones alongside open-back Beyerdynamic headphones]
Essentially, headphones come in two fundamental designs. Headphones come either open-back or closed-back designs, features that emerge as extremely important when it comes to investing in headphones for your auditory experience.
Comparing these two designs is comparing two major functions and hence two different types of people. The closed-back design, which is the most common build of headphones essentially serve one important purpose, which is noise isolation. These headphones have ear-cups that are enclosed with metal and / or strong premium grade plastic and velour or leather pads which in themselves are enough to seal in the music you are listening to and cut out any ambient noise. For noise cancellation even beyond this, there are even noise cancelling closed-back headphones that are a step more than noise isolation. With a closed-back headphone you will not hear the noise that surrounds you, making it appropriate for noise environments like buses, trains and traffic in general. As there is no sound leakage, these headphones allow you to wear them out and about without disturbing fellow travellers or pedestrians. If you enjoy uninterrupted and uncorrupted intimacy with your music, then closed-back headphones are the best for you. The only minor drawback that makes these headphones inappropriate to be used in a commute is the fact that you are so isolated from your surrounding, that you may not hear important signals, such as a car approaching while you are taking a walk on the road.
Open-back headphones, are the exact opposite of their closed-back contemporaries. Just as closed-back headphones provide you with a music-as-clear-as-your-thoughts environment, open-back headphones are designed in a way so that you are also aware of your environment while you have your headphones on. This design allows air to pass through your headphones, making them breathe and giving you an experience like listening to live music, where you are conscious of the sounds and noises around you. What emerges to be the only drawback of such a design is that there is a good amount of sound leakage, making these headphones inappropriate to be worn in public, as it can be disturbing to the people around you. But the same feature makes it a great design as on the road, you will always be aware of what is going on.
Open-back headphones are headphones that can't really be used outdoors because they leak sound. They're best for use for leisure home listening or in a studio for monitoring. You can't record with open-back headphones though.
These two fundamentals of a headphone are tremendously important as when you are investing in a pair, you decide how you want your music to come to you. Both designs are great in their own way; hence the choice lies with you as an individual and your unique and personal music inclinations.