Cloud Computing: It is the use of massively scaled offsite IT resources assembled virtually, accessed over the internet, contracted on demand in real-time or near real-time on a pay-per-use or subscription basis, where the workloads are shared among multiple customers.
Grid Computing: A form of distributed computing whereby a "super and virtual computer" is composed of a cluster of networked, loosely-coupled computers, acting in concert to perform very large tasks.
Utility Computing: The packaging of computing resources, such as computation and storage, as a metered service similar to a traditional public utility such as electricity
Autonomic computing: Computer systems capable of self-management.
Some real stuff
Windows® Azure is a cloud services operating system: http://www.microsoft.com/azure/windowsazure.mspx