Open Back Headphones, as the name suggests, produce more "open" sound. These headphones usually have perforated Earcups which allows air to move around freely inside. Now this air also brings in some outside noise along with it. This is not as bad it sounds, the airy/open sound produced as a result, is more natural surround sounds.Open Back headphones have the ability to enhance sound clarity depending on the type of music you are listening to.
The drivers in Open back headphones emit sound in both directions: towards your ears, and your surroundings. As some amount of sound is allowed to escape freely, you will probably let everyone around you listen to your playlist too.
All it comes down to is what a listener prefers - Bass heavy or natural sound? Some prefer a home theatre sound experience as compared to the isolated studio sound produced by Closed back headphones
Open Back headphones are easily identifiable - Perforated earcups with criss-cross or horizontal outlets for free air movement.