In battle
Hustle increases the user's Attack stat by 50%, but lowers the accuracy of the user's physical moves by 20% (approximated as a multiplier of 3277/4096 from Generation V onwards). OHKO moves, special moves, and status moves are unaffected by Hustle.
Generation IV
Hustle increases self-inflicted confusion damage.
Generation V onwards
Hustle no longer affects confusion damage.
Outside of battle
From Pokémon Emerald onwards, if a Pokémon with Hustle is in the first place in the party (even if fainted), there's a 50% chance that a Pokémon will be forced to the upper bound of their encounter level range. It shares this effect with Pressure and Vital Spirit.