ما هي أركان وواجبات وسنن الحج؟

The pillars of Hajj are four: Ihram, standing at Arafat, Tawaf al-Ziyarah (circumambulation of visiting), and Sa'i (the ritual walk between Safa and Marwah). Its obligations are seven: assuming Ihram from the Miqat, standing at Arafat until sunset for those who stood during the day, spending the nights of the Days of Tashreeq in Mina for those not involved in providing water or care, spending the night in Muzdalifah until after midnight, stoning the Jamarat in sequence, shaving or shortening the hair, and the farewell Tawaf. Everything else is considered a Sunnah (recommended act).

As for the pillars of Umrah, they are three: Ihram, Tawaf, and Sa'i. Its obligations are: shaving or shortening the hair, and assuming Ihram from its Miqat.

The difference between a pillar, an obligation, and a Sunnah: Hajj is not valid without a pillar. Hajj is valid even if an obligation is missed, but a compensatory sacrifice (dam) becomes due. As for a Sunnah, there is no penalty for omitting it.