From Diablo Wiki
Datamine refers to users poking through computer program data and "mining" interesting tidbits. This is a common practice for hackers or aspiring programmers, and when cone to video game code it often serves a beneficial purpose by finding bugs or deeper functions players were not aware of.
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Datamining in Diablo Games
A great deal of information about Diablo and Diablo 2 was unearthed by dataminers from the community, who also found many game bugs and hidden functions or bonuses. Massive info sources likeand full details about bugs and quirks of the were discovered and understood via datamining.
Datamining was used during the Diablo 3 beta test to unearth much content for skills and other features that were not enabled in the beta test version of the game, including the Mystic and many higher level skills and skill runes.
Datamining is not possible on the full version of Diablo 3 since the client-server model keeps much of the game code inaccessible on Battle.net, rather than storing it on the end user's machine where it could be sorted through. Thus tools like item generation calculators can not be constructed for Diablo 3, and [diablo.incgamers.com/blog/comments/diablo-3-legendary-item-drop-rate-survey-results/ players must conduct surveys] to attempt to understand how major game systems like item drops work.
Datamining in Software
Wikipedia.org hosts an extensive article on Datamining if you want more general information.