The Meaning of Umrah
The word umrah in common speech "visit", but in the Shariah it means paying a visit to the Bayt Allah al‑Haram (the Sacred House of God, i.e. the Holy Ka'bah) in a specific form.
The Kinds of Umrah
The Umrah is of two kinds: the first which is performed independently of the Hajj (called al‑Umrat al‑mufradah al‑mustaqillah an al‑Hajj), and the second kind which is performed in conjunction with the Hajj (al‑Umrat al‑mundammah ila al‑Hajj). The al‑Umrat al‑mufradah, the independent Umrah, all the five legal schools agree, can be performed at all times of the year, though it is meritorious to perform it during the month of Rajab according to the Imamiyyah, and in Ramadan according to the four Sunni schools. The time of the conjugate Umrah, which is performed before the Hajj and in the course of the same journey by the Hujjaj coming to the Holy Makkah from distant countries, by consensus of all five schools, extends from Shawwal to Dhul Hijjah. However, there is disagreement among legists about the month of Dhul Hijjah, whether the entire month or only the first ten days belong to the Hajj season. Anyone who performs the conjugate Umrah is considered relieved of the obligation to perform the al‑Umrat al‑mufradah by those who believe in its being obligatory.
Difference Between the Two Kinds of Umrah
The Imamiyyah scholars make a distinction between al‑Umrat al‑mufradah and Umrat al‑tamattu, citing the following reasons:
1. The Tawaf al-nisa' (to be explained later) is obligatory in al‑Umrat al‑mufradah, not in the Umrat al‑tamattu; and according to some jurists is forbidden.
2. The time of Umrat al‑tamattu extends from the first of the month of Shawwal to the ninth of Dhu al‑Hijjah, whereas al‑Umrat al‑mufradah can be performed at all times of the year.
3. The pilgrim (mu'tamir)performing the Umrat al‑tamattu isrequired to shorten his hair (al‑taqsir), whereas the mutamir of al‑Umrat al‑mufradah can choose between shortening his hair or completely shaving his head (al‑halq), as shall be explained later.
4. The Umrat al‑tamattu and the Hajj occur in the same year, which is not the case with al‑Umrat al‑mufradah.
Karrarah, in his book al‑Din wa al‑Hajj ala al‑madhahib al‑'arba'ah, says that, according to the Maliki and Shafii schools, for the mu'tamir of al‑Umrat al‑mufradah all things are permissible, even sexual intercourse, after the shortening of hair (al‑taqsir)or the head shave (al‑halq), irrespective of whether he brings along with him the sacrificial offering (al‑hady)or not. But according to the Hanbali and Hanafi schools, the mu'tamir gets away with al‑taqsir or al‑halq, if he does not bring the sacrificial offering; otherwise he remains in the state ofihram until he gets through the Hajj and the Umrah on the day ofsacrifice (yawm al‑nahr).
The Conditions of the ‘Umrah
The conditions for the Umrah are essentially the same as mentioned in the case of the Hajj.
The Status of Umrah
According to the Hanafi and Maliki schools, the Umrah is not obligatory but a highly recommended sunnah (sunnah mu'akkadah). But according to the Shafii and Hanbali schools and the majority of Imamiyyah legists, it is obligatory (wajib)for one who is mustati, and desirable (mustahabb)for one who is not mustati: In support, they cite the Qur'anic verse:   (Perform the Hajj and the Umrah for Allah.) [17] (Fiqh al‑Sunnah, vol. V; al‑Fiqh ala al‑madhahib al‑'arbaah; al‑Jawahir; al‑Mughni) [18]
The Acts of the Umrah
According to al‑Fiqh ala al‑madhahib al‑'arbaah, whatever is wajibor sunnah for the Hajj is also wajiband sunnah for the Umrah. But the Umrah does differ from the Hajj in certain respects: there is no specific time for performing the Umrah; it does not involve the halt (wuquf)in the plain of Arafat; neither the departure thenceforth to al‑Muzdalifah; nor the ramy al‑jamarat. [19]The Imamiyyah book al‑Jawahir mentions that: "The obligatory acts (af'al or a'mal)ofthe Hajj are twelve: ihram; the wuquf at Arafat; the wuquf at al‑Mash'ar al‑Haram; the entry into Mina; the ramy; the dhibh (sacrifice); its related taqsir or halq; the tawaf (the sevenfold circumambulation of the Ka'bah), and its related raka'at (units of the length of prayers); the sa’y; the tawaf al-nisa', and its related raka'at. The obligatory acts of al‑Umrat al‑mufradah are eight: niyyah (intention); ihram[20] ; tawaf  its related raka'at; the sa’y; the taqsi; the tawaf al-nisa'; and its related raka'at."
This indicates that all the legal schools agree that the acts of the Hajj exceed those of the ‘Umrah by the acts associated with the wuquf. Moreover, the Imamiyyah school considers it obligatory for the performer of the al‑Umrat al‑mufradah to perform a second tawaf, the tawaf al-nisa'. Similarly the Maliki school differs from others in considering halqor taqsir as non‑obligatory for al‑Umrat al‑mufradah.
Two Subsidiary Issues
1. The obligation (wujub)of al‑Umrat al‑mufradah isnot connected with the istitaah for the Hajj. If, supposedly, it is possible for a person to go to Mecca at a time other than that of the Hajj and not possible at the time of the Hajj, then the Umrah instead of the Hajj becomes obligatory for him. If he dies without performing it, its expense is taken out from his heritage." [21]
Similarly, if one has istita'ah for Hajj al‑'ifrad instead of the Umrah, it becomes obligatory upon him; because each of them is independent of the other. This applies to al‑Umrat al‑mufradah. As to Umrat al‑tamattu, which shall be explained later, its wujub depends upon that of the Hajj, since it is a part of it.
2. According to the Imamiyyah, it is not permissible for one intending to enter the Holy Mecca to cross the miqat or enter its haram (sacred precincts) without getting into the state of ihram, even if he has performed the Hajj and the Umrah many times before. Only when the exit and entry recur several times during month, or when after entering the city as a muhrim he goes out any re‑enters for a second time in less than thirty days, it is no obligatory. Therefore, ihram with respect to entry into Mecca is comparable to the wudu' before touching the Holy Qur'an. This clearly demonstrates the baselessness of the lie that the Shiah do not consider al‑Bayt al‑Haram as sacred, and that they pretend to perform the Hajj for the sake of polluting the holy sanctuaries. (!)
According to Abu Hanifah, it is not permissible to go beyond the miqat and enter the haram without ihram, but entry into the remaining area is permissible without ihram. Malik does not agree with this, and two opinions are ascribed to al‑Shafi'i on the matter.
This much of discussion about the Umrah is sufficient for throwing light upon it, so that the reader may grasp its difference with the Hajj, though only in some aspects. What we shall say later will offer further clarification.
  Notes:

[17]. The Qur'an, 2:196.
[18]. According to al‑Mughni, Ahmad ibn Hanbal did not consider the ‘Umrah as being obligatory for Meccans, for the reason that the most important act of the ‘Umrah is tawaf(circumambulation of the Ka'bah) which they do and it suffices them.
[19]. In the book al‑Fiqh 'ala al‑madhahib al‑'arba’ah, it is the author's wont to give the text followed by a commentary and notes. In the text, he states the points of consensus of all the four Sunni schools, the different position of each is given in the commentary. What we have quoted here is taken from the text, not from the commentary.
[20]. According to al‑Din wa al‑Hajj 'ala al‑madhahib al‑'arba'ah, by Karrarah, one of the things which distinguishes the ‘Umrah from the Hajj is that its ihramis not assumed from any of the mawaqitspecified for the Hajj. From the Imamiyyah viewpoint, there is no difference between the miqat for one performing ‘Umrah and the miqat for one on Hajj with regard to ihram.
[21]. The Imamiyyah author of al‑Madarik says: "The better known and sounder of opinions is that the obligation of ‘Umrah is independent of the obligation of Hajj." The author of al‑Jawahir states, "The statements of fuqaha' are not free of confusion... the one which appears sounder is that those who live far away from Mecca are relieved of the obligation of ‘Umrah mufradah, and that which is obligatory upon them is 'Umrat al‑tamattu; whose wujub is related to that of Hajj.