Closed headphones, aka 'sealed' or 'closed-back', have enclosed (non-vented) earcups to isolate you from environmental ambient noises
Closed headphones, also known as sealed headphones, have a solid-back earcup construction preventing noises from leaking into or out of the headphone. Hence, closed headphones can block ambient environmental noise and will also prevent others nearby from hearing your music "leaking" out of the headphones. Closed-back designs may be either full size ('circumaural') or earpad ('supra-aural') types, and all noise cancelling headphones are also closed-back. Audio professionals such as DJs and location recordists and also most portable or mobile listeners will likely choose closed headphones. The sound quality of closed cans has continuously improved in recent years and some now compete directly with top 'open-back' headphones.
For more in-depth discussion on closed/sealed headphones, please see our HeadRoom closed headphone selection guide.