What is Nafs?

Praise be to Allah, the Lord of the Worlds; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
Verily, the soul, or the Spirit, is a thing which Allah has preferred to be from the Unseen i.e. that which only Allah knows. Allah Says: {And they ask you (O Muhammad) concerning the Ruh (the Spirit): say the Ruh (the Spirit) is one of the things, the knowledge of which is only with my Lord. And of knowledge you (mankind) have been given only a little.}[17:85].
The souls are divided into three categories.
1) The secure soul: it is the soul that lives according to the Laws of Allah and remains obedient to Him. As a result the unrest that occupies a soul which indulges in the lusts vanishes from it. Allah the Exalted Says: {O (you) the one in (complete) rest and satisfaction! Come back to your Lord well-pleased (yourself) and well-pleasing (unto Him)!}[89:27-28].
2) The self-reproaching soul: it is the soul that reproaches its owner, when he falls short of the (required) obedience of his Lord Who Says: {And I swear by the self-reproaching person (a believer).} [75:2].
3) The soul that is inclined to evil: it is the soul that disobeys its Lord, surrenders to Satan, and fulfills its desires. Allah Says: {Verily, the (human) self is inclined to evil…}[12:53].

Allah Knows best.