Liquid Easy
"Liquid Easy" is the easiest difficulty. There are plenty of items available, enemy soldiers have a limited range of awareness, and it takes a lot of damage to kill your character.
Naked Normal
"Naked Normal" is a harder difficulty than "Liquid Easy." There are less items available, enemies have a larger range of awareness and your character is more susceptible to attacks.
Solid Normal
"Solid Normal" is very similar to "Naked Normal." The only difference is that the enemies have an even greater range of awareness, which encourages the player to sneak more than he would otherwise.
Big Boss Hard
"Big Boss Hard" is the hardest difficulty level available when you play for the first time. It is like Naked Normal, except that you are weaker and less items are available.
Big Boss Extreme
"Big Boss Extreme" is the hardest difficulty level and can only be unlocked when you beat the game on any of the other difficulty levels. If your character is caught once in the mode, he will certainly die. Additionally, enemy soldiers are at maximum awareness and there are hardly any items available.