Sony MDR 605LP is an example of a very open design. I used to own a pair, and I quite enjoys listening to them (by myself). Seems like today Open-Back Headphones is more popular, they are still open in the back.

The sound of the open air headphones are a bit hard to describe with other words than 'open' :-P It's just a bit more like listening to speakers.

> ... listening to speakers

This is exactly how we hear anything not live. A truly amazing time to be alive.