Is there a difference between naafil, sunnah, mandoob and mustahabb?
Question

                What is the difference between Sunnah ghair mu aqqadah and nawafil when both are voluntary?
Praise be to Allah.
    Sunnah 
    mu’akkadah (confirmed Sunnah), naafil (supererogatory), voluntary and 
    mandoob (recommended) all share a similar meaning; they are acts of worship 
    that are enjoined and encouraged in Islam, without being obligatory. The one 
    who does them will be rewarded but there is no sin on the one who does not 
    do them. 

    That is like 
    praying qiyaam al-layl (optional prayers at night), the sunan rawaatib 
    (regular Sunnah prayers), starting on the right when putting on one’s 
    clothes, and so on. Some of the scholars think that these words are similar 
    in meaning, whilst others – like the Maalikis – differentiate between them. 
    In their view “Sunnah” refers to something that the Prophet (blessings and 
    peace of Allah be upon him) did persistently; naafil refers to things that 
    he did sometimes and not at other times. 

    Ad-Dasooqi 
    al-Maaliki said: Naafil refers in linguistic terms to something extra or 
    additional; in Islamic terminology it refers to that which the Prophet 
    (blessings and peace of Allah be upon him) did but did not do persistently, 
    i.e., sometimes he did not do it and sometimes he did do it. It does not 
    mean that he stopped doing it altogether, because one of his characteristics 
    is that if he did a righteous deed he would not stop doing it altogether 
    after that. … With regard to “Sunnah”, in linguistic terms it refers to a 
    way or path; in Islamic terminology it refers to that which the Prophet 
    (blessings and peace of Allah be upon him) did openly when he was among a 
    group of people, and he persisted in doing it, but there is no proof to 
    indicate that it is obligatory. Sunnahs that are described as mu’akkadah 
    (confirmed) are those that bring a great deal of reward, such as Witr.

    End quote from
    Haashiyat ad-Dasooqi, 1/312 

    Al-Khateeb 
    ash-Sharbeeni ash-Shaafa‘i said: Chapter on offering naafil prayers. Naafil 
    in linguistic terms refers to something extra; in Islamic terminology it 
    refers to actions other than those which are obligatory. They are so called 
    because they are extra to what Allah, may He be exalted, has made 
    obligatory. Naafil is similar to Sunnah, mandoob (recommended), mustahabb 
    (encouraged) and so on. This is the well-known view.

    End quote from
    Mughni al-Muhtaaj, 1/449 

    Ibn an-Najjaar 
    al-Hanbali said: That which is mandoob (recommended) is called Sunnah, 
    mustahabb, naafil, and so on… The highest of that which is recommended is 
    Sunnah, then fadeelah (virtue), then naafilah.

    End quote from
    Sharh al-Kawkab al-Muneer, p. 126 

    It should be 
    noted that the Hanafis regard the one who does not do Sunnah mu’akkadah 
    (confirmed Sunnah) actions as having sinned, but they say that his sin is 
    less serious than that of one who fails to do obligatory (waajib) actions. 

    There is no 
    difference of opinion among the scholars concerning the fact that some 
    Sunnah actions are more confirmed and bring greater reward than others. Thus 
    it may seem that this difference of opinion is the matter of a difference in 
    names only; with regard to the meaning, there is no difference of opinion 
    concerning it. 

    Ibn Nujaym 
    al-Hanafi said: What appears to be the case from the words of our fellow 
    scholars in our madhhab is that sin is incurred by failing to do obligatory 
    (waajib) or sunnah mu’akkadah actions, according to the correct view, 
    because they clearly stated that the one who omits the sunnahs of the five 
    daily prayers is not said to have sinned, but the correct view is that he is 
    sinning. This was stated in Fath al-Qadeer. And they clearly stated 
    that the one who fails to pray in congregation is sinning even though it is 
    a sunnah mu’akkadah according to the correct view. And there are other 
    similar examples to be seen by the one who studies their words. Undoubtedly 
    the sin in some cases is greater than in others, and the sin of one who does 
    not do the sunnah mu’akkadah action is less serious than that of the one who 
    does not do the obligatory (waajib) action. 

    End quote from
    al-Bahr ar-Raa’iq, 1/319 

    And Allah 
    knows best.