There are many differences and interpretations of what both concepts are. Here are a few I find important: * Grid originates from academia, Cloud from the industry. As a consequence the quality, stability and usability of Grid software is a disaster. Grid governance is a mess too (see what happened with Globus two through five...). * Grid cares a lot about security, sometimes so much that it's un-usable in practice (PKIs, complex crypto schemes). Mainstream Cloud security is more reasonable. * Grid is built on old-style web services (WS-*). Most cloud vendors rely on REST-ish services instead. * Grid is by definition cross-organizations. If you want to understand its origins and what it is about you should read two papers by Ian Foster called The Anatomy of the Grid and The Physiology of the Grid. Cloud is almost always single-vendor. * Grid is about diversity, Cloud is about standardization. The advantages of diversity are that you can re-use existing hardware and plug it into the Grid. Its resources are also more diverse: the Grid can offer access to precise hardware at a precise location (think particle accelerators, microscopes, ...). The advantage of standardization is that you can expect things to work most of the time (you'll know what I mean when you will have worked on a large Grid composed of diverse Globus versions, OSs and so on). All in all I would say the Grid is awfully complicated. Globus... well it's a mess and we should just let it die. If it has any kind of future, it is with the Nimbus project: http://www.nimbusproject.org/