The Muḥaddith al-Albānī [d.1419H], may Allāh have mercy upon him, wrote:(1)
al-Rukn (element): (when present) completes the act that it is part of; and when absent, nullifies the act that depends on it. The rukūʿ, for example, is an element in prayer, so if it is absent, the Ṣalāh is nullified.
al-Sharṭ (condition) is similar to the rukn, except it takes place outside the legal act upon which it relies (for its application). For example, without ablution prayer is rendered invalid.
al-Wājib (obligation) has an established command in the Book and the Sunnah. No proof exists that it is either a pillar or condition. The one who performs it is rewarded while the one who deserts it without a valid excuse is punished.
al-Farḍ (duty) is like al-wājib; differentiating between the two is an invented terminology destitute of proof.
al-Sunnah applies to the acts of worship that the Prophet, may Allāh raise his rank and grant him peace, practiced, invariably or occasionally. He has not commanded it as an obligation. The one who performs it is rewarded, while the one who relinquishes it is not punished nor blamed.