Circumaural refers to a material which fully surrounds the outer ear. Typically this would be a good pair of headphones, though you could use this word in reference to ear muffs if you're not afraid of getting hit with a snowball.
The opposite of circumaural would be supra-aural. Supra-aural headphones do not cover the entire ear, are more flimsy and less comfortable.
Both circumaural and supra-aural headphones may be "open" in the back which makes for a more natural listening experience by reducing the sense that the sound is originating from within the listener's head. The drawback to an open style of headphones is that there is more ambient noise flow.
If you want to experience the difference between circumaural and supra-aural before you buy a pair of headphones, place your hands over your ears. First splay your hands loosely, so that your palm is gently covering your outer ear and your fingers are extended but relaxed. Next make your hands into fists and place them over your ears tightly. The first simulates how the circumaural headphones will feel. The second is how the supra-aural will feel. Make sure not to do this experiment in front of other people or else they will surely ridicule you.
Note that a pair of circumaural headphones is also more likely to make your ears uncomfortably warm after prolonged wear. Depending upon your environment and inclinations, this may be a good thing.