According to Wikipedia, Global Sourcing is a term used to describe strategic sourcing in a global setting where the goal is often to exploit global efficiencies in the delivery of a product or service. It's often associated with a centralized procurement strategy for a multi-national wherein the central buying organization attempts to extract an economy of scale through amalgamation and standardization of it's global buying activities.
Although often mistaken for low cost country sourcing, it's not the same as low cost country sourcing is just one aspect of global sourcing. Other instantiations include best cost country sourcing, where an organization tries to take advantage of producers with higher quality or unique services and not just lower labor costs.
For some insights on global sourcing, see the following posts on Sourcing Innovation and Spend Matters.