= "the ability, using a set of open standards and protocols, to gain access to applications and data, processing power, storage capacity and a vast array of other computing resources over the Internet. A Grid is a type of parallel and distributed system that enables the sharing, selection, and aggregation of resources distributed across 'multiple' administrative domains based on their (resources) availability, capacity, performance, cost and users' quality-of-service requirements" (IBM definition, cited at http://en.wikipedia.org/wiki/Grid_computing)
From the Wikipedia article at http://en.wikipedia.org/wiki/Grid_computing
Grid Computing has three aspects:
- Computing resources are not administered centrally.
- Open standards are used.
- Non-trivial quality of service is achieved.
one can classify Grids into several types:
- Computational Grids (including CPU scavenging Grids) which focuses primarily on computationally-intensive operations.
- Data Grids or the controlled sharing and management of large amounts of distributed data.
- Equipment Grids which have a primary piece of equipment e.g. a telescope, and where the surrounding Grid is used to control the equipment remotely and to analyse the data produced.
The Grid IS the future
"Nick Carr’s forthcoming (January 2008) book, “The Big Switch: Our New Digital Destiny.” He makes a clear case that computing resources distributed via vast grids will transform economics, business and culture in the 21st century just as electric utilities did in the last century." (http://blogs.zdnet.com/BTL/?p=6355)
The Grid will be replaced by Virtualization
Bill St. Arnaud says there is no real future for grid computing, and that 'virtualization' will take its place:
"There is however one exception. The Academic Grid is still having lot's of glory thanks to the huge heavily funded European (EGEE) and other US projects. When LHC data will start to be taken at CERN it will reach it's top importance. But, it seems that for other scientific projects Grid Computing is not going to be such a success. It will remain as "Nice to have" but will never replace High-Performance Computing (HPC) on one hand and classical distributed computing tools such as Condor  which exists for more than 20 years on the other hand. Once the governmental fundings will be removed then all the hype of the academic Grid Computing will decline very quickly as well. As was pointed in an interesting talk by Fabrizio Gagliardi about the future of grid computing, at the GridKa07 School, other kinds of Grid Computing infrastructures that will stand on stable financial ground may emerge as the successors, for example Amazon's S3 and EC2 and the joint IBM and Google's cloud computing." (http://billstarnaud.blogspot.com/2007/10/end-of-grid-computing.html)
The Grid will be replaced by Cloud Computing
"Cloud computing is the way forward rather than the Grid. Organizations are not going to be sharing resources [where] you can definitely get resources cheaper from the cloud rather than having to maintain, and then share them with others."
- Savas Parastatidis 
Ian Foster, Carl Kesselman and Steven Tuecke, The Anatomy of the Grid, INT’L. J. SUPERCOMPUTER APPLICATIONS, available at http://www.globus.org/research/papers/anatomy.pdf (describing the next generation of "grid" computing which will provide the technical infrastructure for collective action, group collaboration and virtual firms).