Fear of Sin
The Messilat Yesharim is a book that is a step by step guide how to achieve higher and higher levels of piety.
One of the highest levels of piety is defined as יראת חטא, or fear of sin. At first glance, one might think that this is just another way of describing fear of Hashem.
In actuality, fear of sin is connected with fear of Hashem, but it still reflects a higher level of worship and closeness to G-d.
The lowest level of fear of G-d is really more like fear of punishment. This refers to a person that takes the laws of the Torah seriously. He is motivated by his unwillingness to be guilty of any of the transgressions mentioned in the Torah. He is afraid of the consequences of violating these laws.
A higher level of fear of Hashem is known as יראת הרוממות. This refers to one who is in awe of Hashem’s exalted state. This makes him aware of his own vulnerabilities, and helps keep him in check.
The highest level of fear of sin is an expression of one who already feels extreme closeness to G-d. His fear of sin is an awareness that sin distances a person from Hashem. Just like a Mitzva brings a person closer to Him, the sin makes him more distant. He cannot tolerate the idea that he could do something that would cause a disconnect and distance from Hashem. This is a very high level of observance because it includes a recognition that there is nothing in this world more gratifying than feeling this closeness.
This level of worship is higher than the other two types of fear, because it is based on a deep love for our Creator, that is the cause for this fear of sin.