Everybody has a supposition on what is a Cloud computing. Cloud computing is a modern area emerged by distributed computing that offers many powerful benefits to different organizations. It has an ability to rent a server or a thousand of servers via information technology (IT) services whole the world. The capabilities of Cloud computing are obtained by running a geophysical modeling application on most powerful systems. Organizations can improve their efficiency to quickly and reliably respond to the needs of their customers. It is performed by making a contract for various Cloud services such as applications, software, data storages, and processing capabilities. There are some risks in Cloud-based servers such as maintaining the security of systems, asserting the privacy of information, and insuring the wise expenditures of IT resources. Cloud technology causes the dedicated services to be always on, occurring on the running systems to be upgraded, and accordance with demand to be scaled considerably. Cloud computing involves a range of underlying technologies and configuration options instead of being as a single system. Organizations should consider the strengths and weaknesses of the Cloud technology, service models, and deployment methods through evaluating services to meet their requirements. A Cloud system can extremely store and secure large amounts of data that is accessible only by authorized users and applications. It is supported and sponsored by a Cloud service provider which installs a powerful platform on the Cloud systems. The platform involves some of the required abilities such as the operating system, Apache, a MySQL database, Perl, Python, and PHP with a capability for automatically scaling in response to changing the different workloads. Cloud computing can utilize some applications such as sales automation, email, and forum management on the Internet. Internet can protect data while providing a consumer's service as well it is able to utilize the Cloud storage for holding an application, personal data, and business. A Cloud system with the aid of the Internet can use a small number of Web services in order to integrate maps, photos, and GPS information. This book discusses about the main concepts of Cloud computing. It is an appropriate tutorial for ordinary and professional people to acquire some required information about Cloud technology. Chapter 1 introduces the overall and fundamental characteristic of Cloud systems such as Web services, Grid computing, and hardware virtualization. In Chapter 2, the architectures of Cloud computing including the deployment models and the service models defined for Cloud-based servers are described carefully. Chapter 3 explains various applications of Cloud computing in various applications such as file storage, Cloud database, and email. In Chapter 4, some popular consumer applications designed by Cloud-based systems such as Evernote, iCloud, and Spotify are represented completely. Chapter 5 discusses about the different usages of Cloud servers such Cloud monitoring, healthcare, and banking. In Chapter 6, the security issues of Cloud computing such as privacy, reliability, and compliance are presented carefully. Chapter 7 points out the famous simulation tools designed for Cloud-based issues such as CloudSim, Xen hypervisor, and UEC. Finally, Chapter 8 introduces some well-liked companies established for Cloud-based usages such as CloudLock, CloudMunch, and CloudPhysics. I hope that this book can help to ordinary people and professional researchers to design and implement various applications with Cloud technology. Undoubtedly, this book like any humanistic product is not devoid of any problem. Hence, the next version of this book can be published more appropriate than current version according to valuable suggestions of dear readers. I wish that this book can assist to computer science to design complex systems and to solve some of the exiting problems.