An extremely important and highly reasonable question often posed regarding the terms "nafs" and "rooh" is: "Do these terms dignify one and the same thing or are they two distinctly different entities?" The majority of Islaamic scholars agree that the nafs (soul) and the rooh (spirit) are two names for one and the same thing. However, others maintain that they are two different entities. 
The latter is not a tenable position because it lacks clear, unequivocal delineations of these two terms from the texts of the Qur'aan and Sunnah. Rather, it is a result of a misunderstanding of the terminology in these texts and personal conjecture. This is amply illustrated in the following two examples cited in detail by Imaam Ibnul-Qayyim (may Allaah have mercy on him). 
One group, consisting of some hadeeth scholars, jurists and Soofees, states that "the rooh is other than the nafs." Muqaatil Ibn Sulaymaan explains this view as follows: "Man has life [hayaah], a spirit [rooh] and a soul [nafs]. When he sleeps, his nafs - with which he senses and understands things - emerges from his body; however, it doesn't completely separate from the physical body. Rather, it extends from it, radiating outward like a cable. While both life and the rooh remain in his body (being the two means by which he breathes as well as tosses and turns during sleep), man sees visions by means of the nafs which emerges from him. When he is about to awaken, his nafs returns to him faster than the blinking of an eye. However, if Allaah سبحانه و تعالى wills that he die in his sleep, He seizes that nafs which had come out as described. 
Another sector of hadeeth scholars also holds the opinion that the rooh is other than the nafs but that the nafs, which is in the form of man, is dependent upon the rooh for existence. Man's nature (i.e. nafs) is filled with vanities, desires and passions. It is the source of his trials and afflictions, and there is no enemy more hostile to him than his own nafs. Thus, the nafs wants and loves nothing other than the things of this world, while the rooh longs for the Hereafter and invites to it. 
The two previously stated notions are essentially similar in that they assert that the nafs and the rooh are two separate entities. Other positions exist which are either completely absurd or irrelevant. The absurd views are based on mere personal belief or concepts borrowed from philosophies or teachings foreign to Islaam, such as those stating that the nafs is earthy and fiery, whereas the rooh is luminous and spiritual. The irrelevant theories include the conviction that souls are entities whose nature and reality known only to Allaah سبحانه و تعالى, implying that nothing has been revealed to mankind about them.
In contrast, the correct view, as maintained by the vast majority of Muslim theologians and endorsed by the scholars of Ahlus-Sunnah,  is that the terms "nafs" and "rooh" are interchangeable. However, the term "nafs" is usually applied when the soul is inside the body, and the word "rooh" is used when the soul is apart from the body.
Although these terms may be used interchangeably in relation to their essence, the difference between them is merely difference in attributes and usage. Each one has clearly distinct and restricted applications in certain contexts. For example, the term "nafs" may be used to mean blood as indicated in saying, "Salaat nafsuhu." ("His blood flowed.") Since death resulted from the flowing of one's blood necessitates the exit of one's soul, blood came to be referred to as "nafs." Additionally, the term "nafs" maybe used to mean "the eye" ("'ayn") - commonly referred to as "the evil eye." For instance, it is said, "Asaabat fulaanan nafsun." ("So and so has been struck by an [evil] eye.") 
Upon occasion, the word "nafs" may represent the self (dhaat) as evident in a number of Qur'aanic verses such as the following:
"But when you enter the houses, send upon each other [anfusikum]a greeting of peace - a greeting from Allaah, blessed and good." (Sooratun-Noor, 24:61)
Just as the term "nafs" has several different connotations, so does the term "rooh." It is never used to refer to the physical body (badan) alone or to the soul when it is inside the body. Rather, it has various other usages in the Arabic language and in religious literature. 
In the following words of Allaah سبحانه و تعالى to His Messenger صلى الله عليه وسلم, it is used to mean revelation, specifically, the Qur'aan:
"And thus We revealed to you a rooh [i.e. the Qur'aan] by Our command." (Sooratush-Shooraa', 42:52)
In other places in the Qur'aan the word "rooh" is used to designate Angel Jibreel (Gabriel) عليه السلام, whom Allaah سبحانه و تعالى entrusted with the conveyance of divine revelation. For example:
"Verily, this [Qur'aan] is a revelation of the Lord of the Worlds. Brought down by the trustworthy rooh [i.e. Jibreel عليه السلام]." (Sooratush-Shu'araa', 26:192-193)
The various forces and senses contained in the human body are also spoken of as "spirits." Thus it is said, "ar-rooh al-basir" ("the seeing spirit") and "ar-rooh as-sami'" ("the hearing spirit") and so on. However, these are called "spirits" only by convention. These senses are extinguished upon the death of the physical body, and they are different than the rooh, which does not die or disintegrate.
Finally, the term "rooh" is sometimes used in an extremely restricted sense - to designate the spirit of faith which results from one's knowledge of Allaah سبحانه و تعالى, from turning to Him in repentance and from seeking Him with love and aspiration. This is the spirit (i.e. consciousness of God) with which Allaah سبحانه و تعالى strengthens His obedient chosen servants as stated in the following Qur'aanic verse:
"For those, Allaah has written faith upon their hearts and strengthened them with a rooh from Him." (Sooratul-Mujaadilah, 58:22)
In this manner, knowledge is a "rooh" ("spiritual force"), as is sincerity, truthfulness, repentance, love of Allaah سبحانه و تعالى and complete dependence upon Him. People differ in respect to these types of spiritual forces. Some are so overcome by them that they become "spiritual" beings. Thus is it said, "So and so has spirit." Others lose the power of such spiritual forces, or the greater portion thereof, and thus become earthly, bestial beings. 
About them it may be said, "So and so has no spirit; he is empty like a hollow reed," and so on.
Authentic traditions from the Prophet صلى الله عليه وسلم clearly establish that the rooh and the nafs are essentially one and the same thing. The following narrations, which are two different versions of the same incident, will clarify this point beyond a shadow of a doubt. They explain the manner in which the rooh/nafs departs from the deceased person's body upon death:
Umm Salamah رضي الله عنها reported Allaah's Messenger صلى الله عليه وسلم as saying: "When the rooh is taken out, the eyesight follows it."
Abu Hurayrah رضي الله عنه reported that the Prophet صلى الله عليه وسلم said: "Do you not see that when a person dies his gaze is fixed intently; that occurs when his eyesight follows his nafs [as it comes out]." 
Clearly, since the word "rooh" was used in the first narration and the word "nafs" was used in the second, the two terms are, in essence, interchangeable.  
 See Ibn al-Aloosi's رحمه الله Jalaa' al-'Aynayn, pp. 142-143 and as-Safareeni's رحمه الله Lawaami' al-Anwaar, vol. 2, pp. 31-32.
 For a more detailed account of the various contradictory opinions, see Kitaab ar-Rooh, pp. 296-297.
 Paraphrased from Imaam Ibnul-Qayyim's رحمه الله Kitaab ar-Rooh, p. 296.
 See Kitaab ar-Rooh, pp. 294-297 and Jalaa' al-'Aynayn, pp. 142-143.
 See Lane's Lexicon, vol. 2, p. 2828.
 See at-Tahaawiyyah, pp. 444-445 and Kitaab ar-Rooh, pp. 295-296
 For more details, see Lawaami' al-Anwaar, pp. 31-32; at-Tahaawiyyah, p. 445 and Kitaab ar-Rooh, p. 297.
 Both of the preceding hadeeths are authentic and were related by Imaam Muslim رحمه الله. See also Imaam al-Qurtubi's رحمه الله at-Tadhkirah, p. 70.
 See also Shaykh Siddeeq Hasan Khaan's رحمه الله Fat-hul-Bayaan, vol. 8, p. 232.
 Abu Bilal Mustafa al-Kanadi رحمه الله: Mysteries of the Soul Expounded