“Global city” is a term popularized by the sociologist Saskia Sassen* when she used it in the title of her 1991 book: “The Global City: New York London Tokyo”.
The term “Megacity” (Ger., Eng.) is used to describe urban areas that reach a certain magnitude or expansion, normally when the amount of inhabitants exceeds 10 million. (Here’s a top twenty list in German.) Megacities can be vulnerable, the “Mega Cities Task Force” has information about this and further links, I hope it is still working.
Global Cities (Ger., Eng.) can be large too, but their importance originates in their economic, political and cultural significance they have within a growing global system: They are not only passively shaped by this system, but they actively form it and play a role within this process of globalization. (For a conference about this topic go here.) A “hit list” of Globalcities was put together 2008 by the GaWC Research Network, the latest list is the 2010 Global Cities Index (long version here), thrown together by the magazine Foreign Policy. The first and second rank in all lists go to NewYork City and London, the following vary according to the special emphasis the listmaker put on various factors.
* See here for an interesting article about her and her family, sorry Ger. only.