What is a grid and what is a cloud?
Cloud computing is user friendly grid computing. Take any grid and say: Lets us make the life of the user easier. first, no need to learn the submission language and scripts. Second, no need to wait anxiously to see if your job runs, or having a nightmare finding out WHEN the job will run.
Third the user always worries "Do I have resources to run my job?". Users less sophisticated sit by side and watch. They do not dare to use the grid, because a too complicated learning curve is the driving test for mainstream users
Cloud Computing delivers all apps from what it was a grid, as a service. Just a job submission interface and a predictable response time according to a Service Level Agreement. A cloud is nothing but an elastic grid with Service Level Agreement implementation capability and charge-back and or billing capability. All all users feel no discrimination and can use the cloud service at ease. They use and they pay only for what they use.
Once they pay, they can demand the service promised to maintain their standards. This user right simply does not exist in grid where the ivory tower policy makers decide what you are entitled to receive every day and every time of the day, depending on your assigned priority and what other users are in the system.