Cloud Computing is For Service Oriented where as Grid Computing is for Application Oriented. Grid computing is used to build Virtual supercomputer using a middler ware to achieve a common task that can be shared among several resources. most probably this task will be kind of computing or data storage.
Cloud computing is providing services over the internet through several servers uses Virtualization.In cloud computing either you can provide service in three types Iaas , Paas, Saas .
This will give you solution when you don't have any resources for a short time Business service over the Internet.
