In a nutshell,
They went at it, De La Hoya dominated the first bunch of rounds, Mosley took over the domination, then (to my POV) De La Hoya took back the domination in the last rounds.
De La Hoya ended up with "way better" stats on %/Numbers of punch landed. The 3 judges ruled Mosley winner at the surprise of Mosley/De La Hoya and the anouncers of PPV.
Punch stats showed De La Hoya landed 221 punches to 127 for Mosley.
Mosley's head accidentaly hit De La Hoya's causing a cut, it's said that the judges "scored the blood" more than the fighting.