Cloud computing is an up-coming commercial infrastructure paradigm. clouds promise to eliminate the need for maintaining cost-effective computing facilities by companies, organizations, institutes etc., This is achieved through the use of virtualization of memory and resource time sharing. Clouds serve to address with the same shared set of physical resources a large user base with different needs. Thus, clouds have the possibility to provide to their owners the benefits of economy of scale. At the same time, clouds promise for scientists an alternative to clusters, grids and supercomputers.