Difficulties are a modification to the base Necrotic experience tables. Each difficulty has a multiplier applied to it, and all values are rounded to the nearest whole number.
Difficulties cannot be changed, and currently do not provide any benefits to hard modes - this however is open to modification.
Most of the skills (except Dungeoneering) are based off their RS exp values, and are then multiplied by a skill value, and finally multiplied by a difficulty value.
The Regular difficulty was created as the control difficulty, which the main game is based around.
The Easy difficulty is (regular * 1.25), for a 25% EXP increase compared to Regular mode.
The Hard difficulty is (regular * 0.75), for a 25% EXP decrease compared to Regular mode.
The Extreme difficulty is (regular * 0.5), for a 50% EXP decrease compared to Regular mode.
See the table for examples.