I want to know the difference between a rukn (pillar) and a sunnah.

A rukn (pillar/integral component) is something without which a thing cannot exist. Whoever intentionally abandons it commits a sin, and their act of worship is invalid. If one abandons it out of forgetfulness or ignorance, their act is still invalid, but they do not incur sin. As for Sunnah (definition), it refers to that which is desirable (mustahabb) and supererogatory (masnūn), meaning what the Lawgiver commanded without obligation. Whoever abandons it does not sin, and it does not affect the validity of their worship, but they miss out on the reward. Sunnah also refers to a way or method, as in the Prophet’s (peace be upon him) saying: "You must adhere to my Sunnah and the Sunnah of the rightly guided Caliphs. Cling to it with your molars." It is also used to denote a prescribed way to be followed, whether it is obligatory or not. Among jurists, its usage predominantly refers to that which is not obligatory, and it is used in contrast to bid‘ah (innovation).