To the best of my knowledge Enumeration is a throwback to older Java versions. At some point there was the decision to move to the Iterator pattern (this may have had something to do with the GOF book, not sure). However to keep the 'not breaking backwards compatability' contract the Enumerations were left in. I don't think there is a big difference, although Iterator is the 'prefered' way to go.
My memory is sketchy on this so I may be off.
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