Hajj is one of the five pillars of Islam, which form the basic foundations of a Muslim’s beliefs. Unlike prayer, alms and fasting, Hajj is required of every Muslim only once during their lifetime, so long as they are physically, emotionally, and financially eligible. However, Muslims are permitted to go more than once if they have the means.
The Hajj pilgrimage is a test of patience and temperament, and can be spiritually, emotionally, and physically challenging. However the sanctity of this occasion provides an experience and opportunity like no other for drawing closer to Allah (SWT), and many Muslims around the world strive their whole lives for the opportunity to perform Hajj.
Muslims believe Hajj offers a chance to wipe clean past sins and start anew before Allah. Additionally, the act of performing Hajj with increased Taqwa (being God-conscious) and to a high standard of perfection, is rewarded with Paradise.

A Mabroor Hajj, complete and accepted, has no reward for it but Jannah.
Hadith | Muslim
While Hajj is first and foremost a spiritual duty, it is also very physically demanding. As pilgrims walk on average between 5km and 15km per day, it is encouraged for Muslims to physically prepare for Hajj weeks in advance, so that the travel that is required between locations across the five or six-day pilgrimage is not a shock to the system. Being physically prepared provides an aide to the emotionally and spiritually demanding aspects of Hajj.
Hajj and Umrah are both sacred pilgrimages in Islam and share many of the same qualities, rites and rituals. However, there are some main differences, one of the biggest being that Hajj is a fifth pillar of Islam and required of every Muslim so long as they have the capacity, whereas Umrah is a voluntary pilgrimage.
Here are some more differences between Hajj and Umrah:
In 628 CE, Prophet Muhammad (peace be upon him) and the Muslims performed the first Hajj. He was instructed to re-establish the traditions started by Prophet Ibrahim (peace be upon him) as instructed by Allah (SWT), generations before.
Under the commandment of Allah, Ibrahim (peace be upon him) travelled to the barren desert of Makkah to perform various acts of worship, including building the Kaaba.