What is the difference between the soul (ruh) and the self (nafs)?

The nafs (soul) is understood to be the self, identity and the core of human existence (consciousness, awareness, desires). The ruh (spirit) is regarded as the life-force or the breath of life (as it were), meaning that which allows the being to live. Soul and Spirit are therefore not same thing. The Soul is Nafs. The Spirit is Ru'h. That which has been mentioned in Surah Isra is the ruh: "And they ask thee about the ruh. Say, the ruh is by the command of my Lord and they have been given very little knowledge of it" (Qur'an Isra:85) It is of this element that we lack knowledge. Of the nafs, however, we have some knowledge, quite extensive actually. The word nafs, and its derivations (anfusihim, anfusikun, nafsikum, nafisa, nifaas) have been scattered throughout the Qur'an. In Arabic, al-Ilm al-nafs, loosely translated as Knowledge of the Soul, is defined in secular sciences as Psychology, although not in the same regard.

The soul is from Allah and is unchanging. You could call it the spark of life The nafs is from self, and is ever changing. It is this that you are "developing" in your self-discipline and automatic habits.