An emerging internet based super computing model is represented by cloud computing. Cloud computing is the convergence and evolution of several concepts from virtualization, distributed storage, grid, and automation management to enable a more flexible approach for deploying and scaling applications. However, cloud computing moves the application software and databases to the large data centers, where the management of the data and services may not be fully trustworthy. The concept of cloud computing on the basis of the various definitions available in the industry and the characteristics of cloud computing are being analyzed in this paper. The paper also describes the main cloud service providers and their products followed by primary cloud computing operating systems.