Cloud computing is a natural evolution of concepts in Grid computing.
With very basic terms:
Grid says: “Let’s join our domains and efforts by shaing your resurces in order to get more computational power”.
Cloud says: “We can provide you more computational power than what you need. Just tell us what you want and we will give it to you”.
With more technical terms:
-Grid computing (computational grid) is the application of several computers to a single problem at the same time — usually to a scientific or technical problem that requires a great number of computer processing cycles or access to large amounts of data. Users mostly need to have knowledge of technology infrastructure to utilize the Grid services.
-Cloud computing is Internet (“cloud”) based development and use of computer technology (“computing”). It is a style of computing in which dynamically scalable and often virtualised resources are provided as a service over the Internet. Users need not have knowledge of, expertise in, or control over the technology infrastructure “in the cloud” that supports them.