The lesser pilgrimage is the same set of rites as part of the Hajj, but its recent history is administrative rather than devotional, and it is where Saudi policy on pilgrimage is most visible.
The rite
Umrah is entering ihram, tawaf around the Kaaba, sai between Safa and Marwa, and then shaving or shortening the hair. It can be performed at any time of year, takes a few hours, and does not discharge the Hajj obligation. Its legal status divides the schools: Shafii and Hanbali jurists hold it obligatory once in a lifetime, Hanafi and Maliki jurists strongly recommended but not required.
Where Hajj is capped and Umrah is opened
The Hajj is rationed by quota and cannot be enlarged without enlarging the crowd at a single fixed season. Umrah has no such ceiling, and the Saudi state has opened it progressively wider over the last decade on stated economic policy, so that it now runs year-round at a volume the Hajj could not accommodate. The same period has seen the demolition of a great deal of Ottoman-era and early Islamic fabric in Mecca to build the high-rise accommodation the traffic requires, a loss that Muslim heritage bodies have objected to publicly and without success.