From the hitman blood money somethingawful thread about the difficulty settings in the game.
Difficulty Settings
* Rookie: Being a rookie gets you infinite saves, an accommodating A.I., and full Agency assistance. You don't have to worry about Agent 47's notoriety at this level.
* Normal: At normal difficulty, you get seven saves per level, regular A.I. behavior, and normal Agency assistance. Agent 47's notoriety level will affect his world.
* Expert: Only three saves per level. The A.I. hits harder, and the Agency helps less. 47 is a victim of his own notoriety, and the evidence he leaves behind will start to haunt him.
* Pro: No saves. The A.I. goes to the wall, the Agency treats you like a rented mule, and your actions will come back to bite you. Leave evidence behind and you might as well wear a target.
The 'Agency help' refers to the help given at the top of the screen. Sorry the original source for this is unknown.