What is the difference between a pillar (rukn) and a condition (shart) in prayer?

The conditions (Shurut) of prayer are matters external to its essence, such as purity (Taharah), covering the 'awrah (modesty), and facing the Qiblah. The prayer is not valid unless these are fulfilled. As for the pillars (Arkan), they are the essential elements within the essence of the prayer itself, such as bowing (Ruku'), prostration (Sujud), and standing (Qiyam). If either the conditions or the pillars are absent, the essence of the prayer becomes void.