Alot of stuff was added and a few things were removed. Of the top of my head, I can't think of them all but it's a somewhat different film than the theatrical version.
The focus is different. I do know that i prefer this cut instead of the original.
Minor changes to scenes, smal additions, and some alternate takes/angles. Overall, it adds to the atmosphere of the picture, giving it alittle more breathing room, character development, and underlining the political FBI stuff a little bit more. Pretty much this is what would happen if you let Michael Mann tinker with the film in the editing room for a couple more months. Both cuts are largely the same, overall. If you own neither, I would recommend the director's cut, as it offers a slightly fuller film. TC is 157 mins, this is 165 mins.
Director's cut features a 30-minute making of piece and a Mann commentary that focuses almost exclusively on the historical aspects of the film. (Mann typically only discusses content, rather than style, in his commentaries, and since this is historically based he basically talks about Muhammed Ali for 3 hours)
I had a hell of a time finding out info about this myself; feel free to ask more questions.