Whether the Umrah is obligatory is one of the clearest worked examples of how the four Sunni schools differ, and it is worth setting out because the disagreement is about method rather than about piety.
The Shafii and Hanbali positions hold the Umrah wajib, obligatory once in a lifetime for those able. They rely principally on the Quranic instruction to complete the Hajj and the Umrah for God, reading the two as coupled in the same command, and on reports in which the Prophet is asked whether Umrah is obligatory and does not deny it.
The Hanafi and Maliki positions hold it sunna muakkada, strongly recommended but not required. They read the Quranic verse as governing the completion of a pilgrimage once begun rather than as imposing the pilgrimage itself, they note that the hadith listing the pillars of Islam names the Hajj alone, and they treat the reports relied on by the other schools as not reaching the standard needed to establish an obligation.
Nobody in this argument thinks the Umrah is unimportant. The dispute is about what weight a coupling in a verse can carry and what grade of report can create a duty.
Umrah is also performed as part of the Hajj. In the tamattu form the pilgrim performs Umrah first, leaves ihram, and re enters it for the Hajj; in qiran the two are combined in one state of ihram; in ifrad the Hajj is performed alone. Which form a pilgrim adopts determines whether an animal offering is required.
The rite itself is short: ihram, tawaf, sai, then halq or taqsir, shaving or shortening the hair. It can be done in a few hours at any time of year, and it does not discharge the Hajj obligation.