A: Yes, there are differences.
The Nafilah is a desirable supererogatory act, while the Fard is obligatory, based on a definitive text. If one does not offer the Nafilah Prayer, such as Salat-ul-Duha (supererogatory Prayer before noon), Al-Witr (Prayer with an odd number of units), or the Sunnah Ratibah (supererogatory Prayer performed on a regular basis) related to the the Zhuhr (Noon), Fajr (Dawn), Maghrib (Sunset), and ‘Isha’ (Night) Prayers, one is not regarded as sinful for this. On the other hand, if one gives up the the Fard Prayers, one is sinful and may even be considered a disbeliever if one does so intentionally.
The Nafilah Prayer may be offered while sitting, even if the person performing it is healthy, while the Fard Prayer cannot be performed while sitting unless the person offering it has a reason for this; such as being sick or disabled.
The Prophet (sallallahu alaihi wasallam) would sometimes offer the optional late night Prayer while sitting, and he (sallallahu alaihi wasallam) said: (Whoever prays while standing will get greater reward;one who prays while sitting will get half of the reward of the person praying while standing; and one who prays laying will get half of the reward of the person who prays sitting.) Peace and blessings be upon the Prophet.
So, it is permissible in general to pray the Nafilah during the night or day while sitting. However, this is not the case with the Fard Prayer. The latter must be offered while standing, as the Prophet (sallallahu alaihi wasallam) said to “Imran: (Offer Prayer while standing). He said to ‘Imran ibn Al-Husain (may Allah be pleased with him): (Pray while standing and if you cannot, pray while sitting, and if you cannot do even that, then pray lying on your side.) (If you cannot do even that, you can pray lying on your back.) This Hadith is about the Fard; as for the Nafilah, there is no problem if one performs it while sitting; if he or she finds himself or herself too tired to stand while praying the same, they may sit.
(Part No. 10; Page No. 385)
Source: English Translations of Collection of “Noor ala Al-Darb” Programs, Volume 10. By: Sheikh `Abdul `Aziz Bin `Abdullah ibn `AbdulRahman ibn Bazz (May Allah forgive and reward al-Firdouse to him and his parents).He was The Mufti of Kingdom of Saudi Arabia, Chairman of the Council of Senior Scholars, and Chairman of Department of Scholarly Research and Ifta’.
This English Translations are collected from alifta.com (website is down since longtime), Portal of the General Presidency