A global city is a city which is seen to be important in the global economic system.
It is seen as beneficial to be classed as a global city so many groups have tried to create systems to classify and rank which cities are 'world cities' and which are 'non-world cities'. Some of these lists can be found on the Wikipedia - Global City page.
Two Indian cities which appear in some of the global city rankings are Mumbai and Delhi.
Find more information about these cities by following the links on this page.