At the core of cloud computing is a simple concept: software as a service, or SaaS. Whether the underlying software is an application, application component, platform, framework, environment, or some other soft infrastructure for composing applications to be delivered as a service on the Web, it's all software in the end. But the simplicity ends there. Just a step away from that core, a complex concoction of paradigms, concepts, and technologies envelop cloud computing.