Informational summary, not a religious ruling. Where the schools of law differ, both positions are named. For anything affecting the validity of your Hajj, ask a qualified scholar or your group's guide.
Miss one of these and the Hajj is not valid. No sacrifice repairs it. The schools count them differently, so each entry below says where they diverge rather than presenting one position as settled.
Ihram
إحرامEntering the consecrated state, with the intention
Ihram is not the cloth; it is the state you enter, marked by the intention (niyyah) and the talbiyah. The two unstitched white sheets a man wears are the outward sign of it. Women wear ordinary modest clothing and do not cover the face with a fitted veil. Ihram is entered at or before the miqat, and the restrictions begin at that moment.
Between the schools: Agreed as a rukn by all four schools, though some classify the niyyah as the rukn and the garments as a condition.
Wuquf at Arafah
وقوف عرفةStanding at Arafah
Being present in the plain of Arafah for any part of the period from midday on 9 Dhul Hijjah until dawn on the 10th. This is the one rite with no substitute: “Hajj is Arafah.” A pilgrim who misses it has missed the Hajj, regardless of what else they performed.
Between the schools: Agreed as a rukn by all four schools.
Tawaf al-Ifadah
طواف الإفاضةThe tawaf of the return, after Arafah
Seven circuits of the Kaaba, anticlockwise, performed after returning from Muzdalifah. It may be delayed but it cannot be omitted, and until it is performed the intimacy restriction of ihram remains in force.
Between the schools: Agreed as a rukn by all four schools.
Sa‘i
سعيWalking between Safa and Marwah
Seven passages between the hills of Safa and Marwah, beginning at Safa and ending at Marwah, retracing Hajar’s search for water for her son Isma‘il.
Between the schools: A rukn in the Shafi‘i, Maliki and Hanbali schools. The Hanafi school classifies it as wajib rather than a rukn, meaning its omission is compensated by a sacrifice rather than invalidating the Hajj.
Halq or taqsir
حلق / تقصيرShaving or shortening the hair
Men shave the head or shorten the hair throughout; shaving carries the greater reward. Women shorten by roughly a fingertip’s length from the end of the hair, and do not shave.
Between the schools: A rukn in the Shafi‘i school. Wajib in the Hanafi, Maliki and Hanbali schools.
Tartib
ترتيبPerforming them in the correct order
The sequence itself is part of the rite: ihram before everything, Arafah before the tawaf of return, and so on.
Between the schools: Counted as a rukn by the Shafi‘i school. The others treat correct order as a condition rather than a separate pillar.
Omitting one of these does not invalidate the Hajj, but it requires a dam (a sacrifice) in compensation. The enumeration below follows the Hanafi school, which is what most pilgrims from South and Southeast Asia are taught; the others distribute several of these differently.
Nothing is owed for omitting any of these.