What is the difference between open and closed headphones?

i know what the difference is i the way it looks but how does it actually affect sound and which one is better to use in a recording studio?
Update: tie breaker anyone?
Update 2: sorry i didnt specify, i wouldnt be using the headphones for recording really, but more for mixing (and obviously i would do a final mixdown over speakers) so i think im gonna go with open headphones unless anyone objects?
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