This document provides an overview of cloud computing, including definitions, models, and applications. It begins with definitions of cloud computing from various experts and the National Institute of Standards and Technology (NIST). It describes the essential characteristics, service models (SaaS, PaaS, IaaS), and deployment models of cloud computing. The document discusses the differences between clusters, grids, and clouds. It provides examples of cloud applications like ECG analysis, protein structure prediction, and social networking. Finally, it outlines the actors in cloud computing reference architecture like cloud providers, consumers, brokers and carriers along with example usage scenarios.