It should be stressed, first of all, that Hajj and `Umrah are among the most important means for the expiation of sins. The Prophet (peace and blessings be upon him) is reported to have said, “Alternate between Hajj and `Umrah because both rid one of poverty and sins just as the blacksmith’s bellows remove all impurities from metals like iron, gold and silver. The reward for Hajj mabrur is nothing short of Paradise” (At-Tirmidhi).
In his response to this question, Sheikh Ahmad Kutty, a senior lecturer and Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states the following:
There are a number of differences between Hajj and `Umrah; let me list the major differences:
1. Hajj is the fifth pillar of Islam, and it is mandatory for everyone who is physically and financially able to perform it once in a lifetime. `Umrah is not a pillar of Islam and it is only recommended and not obligatory.
2. Hajj must be performed in the prescribed time period, namely the months of Hajj, and even more specifically, the major rites are done in the first two weeks of Dhul- Hijjah. `Umrah, however, can be done any time of the year.
3. `Umrah involves only the rites of Tawaf and Sa`i with ihram; whereas Hajj involves staying in Mina, performing the rite of wuquf (standing) in the plains of `Arafat, staying in Muzdalifah, and pelting the stone pillars, as well as sacrifice in some cases. `Umrah, however, does not involve any of these.