from " I. Foster, C. Kesselman. -The Grid: Blueprint for a New Computing Infrastructure": "A computational grid is a hardware and software infrastructure that provides dependable, consistent, pervasive, and inexpensive access to high-end computational capabilities."
An infrastructure of computing resources, such as storage and processing units, that is transparently made available to the user through a network. The term "grid" originates from the analogy of the electric grid, as the idea is to make computing just as ubiquitous and easy to use.
A distributed computing architecture that will enable virtual (i.e. independent of the physical location and time zone distribution of its members) teams of researchers to collaborate on the bigger challenges in many disciplines.