The "Umrah (Arabic: عمرة ‎‎ )" is a pilgrimage to Makkah, Saudi Arabia, performed by Muslims that can be undertaken at any time of the year, in contrast to the Haj. In Arabic, Umrah means "to visit a populated place." In the Sharia, Umrah means to perform Tawaf round the Kaaba and Sa'i between Al-Safa and Al-Marwah, after assuming Ihram (a sacred state), either from a Miqat like Zu 'l-Hulafa, Juhfa, Qarnu 'l-Manazil, Yalamlam, Zat-i-'Irq, Ibrahim Mursia, or a place in Hill. It is sometimes called the 'minor pilgrimage' or 'lesser pilgrimage'.

Performing Umrah is neither imperative nor obligatory, but it is a privilege and performing it even once in a lifetime can bring a pilgrim great rewards.

Though a smaller scale pilgrimage than Haj, Umrah is just as sacred and just as impressive. It is also a more flexible option for those struggling with certain commitments and time or financial constraints.
Unlike Haj, you don't have a fixed time or date by which you have to perform Umrah. You can complete it at any time of year, either in conjunction with Haj (Umrat al-tammatu) or independent of it (al-Umrat al mufradah).
Umrah is a mini Haj that can be performed at any time of the year. It can be performed along with Haj and in other days as well. According to Quran:

"---accomplish Haj and Umrah to please Allah". (Al-Baqrah, 196)

The Holy Prophet (peace be upon him) has described the great merits of performing Umrah.He has said:

"The people who come to perform Haj or Umrah are the guests of Allah because they visit His House on His Call; therefore, Allah grants them what they ask for"

He has also said:

"One Umrah becomes a recompense for the sins committed up to the next Umrah"

He has said again:

"Anyone who leaves his house with the intention of performing Haj or Umrah and dies on the way will enter paradise without judgement. Allah is proud of those who perform tawaf of Baitullah"

Performing Umrah during Ramadan is most commendable and equal to Haj in merit and excellence. But this act does not substitute the performance of Haj which is Fard (imperative), once in a life time if one is financially and physically capable.