This article discusses the concept of Cloud Computing to achieve a complete definition of what a Cloud is, using the main characteristics typically associated with this paradigm. Over 20 definitions have been studied to extract a consensus definition and a minimum definition containing the essential characteristics. The paper pays much attention to the Grid paradigm, as it is often confused with Cloud technologies. It also describes the relationships and distinctions between the Grid and Cloud approaches. The paper presents an overview of the Cloud scenario, analyzes existing Cloud definitions, and extracts relevant features to form both an integrative and a basic Cloud definition. It then compares Clouds and Grids, highlighting their similarities and differences. Key distinctions include resource sharing, heterogeneity, virtualization, security, high-level services, architecture, software workflow, scalability, usability, standardization, and payment models. Clouds are characterized by their ability to provide virtualized resources that can be dynamically reconfigured to adjust to variable loads, allowing for optimal resource utilization. They are typically exploited through a pay-per-use model with customized SLAs. In contrast, Grids focus on resource sharing across organizations and provide a more limited set of high-level services. Clouds are more user-friendly, scalable, and offer better usability, while Grids require more technical expertise. The paper concludes that Cloud Computing is still evolving and lacks a clear, complete definition. However, a unified definition is essential to determine research areas and explore new applications. Virtualization is a key enabler of Clouds, enabling features such as on-demand resource sharing and security through isolation. Usability and standardization are also important for Clouds to achieve widespread adoption. The paper emphasizes the need for further research and standardization to ensure interoperability and address challenges such as scalability and security.This article discusses the concept of Cloud Computing to achieve a complete definition of what a Cloud is, using the main characteristics typically associated with this paradigm. Over 20 definitions have been studied to extract a consensus definition and a minimum definition containing the essential characteristics. The paper pays much attention to the Grid paradigm, as it is often confused with Cloud technologies. It also describes the relationships and distinctions between the Grid and Cloud approaches. The paper presents an overview of the Cloud scenario, analyzes existing Cloud definitions, and extracts relevant features to form both an integrative and a basic Cloud definition. It then compares Clouds and Grids, highlighting their similarities and differences. Key distinctions include resource sharing, heterogeneity, virtualization, security, high-level services, architecture, software workflow, scalability, usability, standardization, and payment models. Clouds are characterized by their ability to provide virtualized resources that can be dynamically reconfigured to adjust to variable loads, allowing for optimal resource utilization. They are typically exploited through a pay-per-use model with customized SLAs. In contrast, Grids focus on resource sharing across organizations and provide a more limited set of high-level services. Clouds are more user-friendly, scalable, and offer better usability, while Grids require more technical expertise. The paper concludes that Cloud Computing is still evolving and lacks a clear, complete definition. However, a unified definition is essential to determine research areas and explore new applications. Virtualization is a key enabler of Clouds, enabling features such as on-demand resource sharing and security through isolation. Usability and standardization are also important for Clouds to achieve widespread adoption. The paper emphasizes the need for further research and standardization to ensure interoperability and address challenges such as scalability and security.