Enterprises continuously seek innovative approaches to reduce operational computing costs while getting the most from their resources. Recent developments in Cloud Computing technology play a major role in helping organizations to reduce the operational cost. It is a model for enabling convenient, on-demand network access to a shared pool of configurable computing resources (e.g., networks, servers, storage, applications, and services) that can be rapidly provisioned and released with minimal management effort or service provider interaction.

Figure 1 presents an overview of the NIST Cloud Computing reference architecture [1], which identifies the major actors, their activities and functions in Cloud Computing.

IT experts have classified Cloud Computing vendors into three broad categories based on the fundamental nature of the Cloud-based solution they provide:

The main difference between these service models lies in how responsibilities are divided between Cloud Service Provider (CSP) and Cloud Consumer. For example, In IaaS offering, the Cloud consumer has extensive control over his servers and the installed operating systems and applications; the virtualization infrastructure and at least parts of the network infrastructure. With SaaS, in contrast, the Cloud consumer usually controls only certain configuration parameters of the contracted service the application and all underlying infrastructure is under control of the CSP. PaaS lies between these two extremes in that the customer controls the application as a whole (including the code), while the CSP controls the runtime environment and supporting infrastructure.