一、云计算定义
云计算是一种模型，它可以实现随时随地，便捷地，随需应变地从可配置计算资源共享池中获取所需的资源（例如，网络、服务器、存储、应用、及服务），资源能够快速供应并释放，使管理资源的工作量和与服务提供商的交互减小到最低限度。
Cloud computing is a model for enabling ubiquitous, convenient, on-demand network access to a shared pool of configurable computing resources (e.g., networks, servers, storage, applications, and services) that can be rapidly provisioned and released with minimal management effort or service provider interaction. This cloud model is composed of five essential characteristics, three service models, and four deployment models.
二、基本特点
2.1、随需应变的自助服务
消费者可以单方面地按需自动获取计算能力，如服务器时间和网络存储，从而免去了与每个服务提供者进行交互的过程。
On-demand self-service. A consumer can unilaterally provision computing capabilities, such as server time and network storage, as needed automatically without requiring human interaction with each service provider.
2.2、无处不在的网络访问
网络中提供许多可用功能，可通过各种统一的标准机制从多样化的瘦客户端或者胖客户端平台获取（例如，移动电话、笔记本电脑、或PDA掌上电脑）。
Broad network access. Capabilities are available over the network and accessed through standard mechanisms that promote use by heterogeneous thin or thick client platforms (e.g., mobile phones, tablets, laptops, and workstations).
2.3、资源共享池
服务提供者将计算资源汇集到资源池中，通过多租户模式共享给多个消费者，根据消费者的需求对不同的物理资源和虚拟资源进行动态分配或重分配。资源的所在地具有保密性，消费者通常不知道资源的确切位置，也无力控制资源的分配，但是可以指定较精确的概要位置（如，国家、省、或数据中心）。资源类型包括存储、处理、内存、带宽、和虚拟机等。
Resource pooling. The provider’s computing resources are pooled to serve multiple consumers using a multi-tenant model, with different physical and virtual resources dynamically assigned and reassigned according to consumer demand. There is a sense of location independence in that the customer generally has no control or knowledge over the exact location of the provided resources but may be able to specify location at a higher level of abstraction (e.g., country, state, or datacenter). Examples of resources include storage, processing, memory, and network bandwidth.
2.4、快速而灵活
能够快速而灵活地提供各种功能以实现扩展，并且可以快速释放资源来实现收缩。对消费者来说，可取用的功能是应有尽有的，并且可以在任何时间进行任意数量的购买。
Rapid elasticity. Capabilities can be elastically provisioned and released, in some cases automatically, to scale rapidly outward and inward commensurate with demand. To the consumer, the capabilities available for provisioning often appear to be unlimited and can be appropriated in any quantity at any time.
2.5、计量付费服务
云系统利用一种计量功能（通常是通过一个付费使用的业务模式）来自动调控和优化资源利用，根据不同的服务类型按照合适的度量指标进行计量（如，存储、处理、带宽和活跃用户账户）。监控，控制和报告资源使用情况，提升服务提供者和服务消费者的透明度。
Measured service. Cloud systems automatically control and optimize resource use by leveraging a metering capability1 at some level of abstraction appropriate to the type of service (e.g., storage, processing, bandwidth, and active user accounts). Resource usage can be monitored, controlled, and reported, providing transparency for both the provider and consumer of the utilized service.
三、服务模型
3.1、软件即服务（SaaS）
该模式的云服务，是在云基础设施上运行的、由提供者提供的应用程序。这些应用程序可以被各种不同的客户端设备，通过像Web浏览器（例如，基于Web的电子邮件）这样的瘦客户端界面，所访问。消费者不直接管理或控制底层云基础设施，包括网络，服务器，操作系统，存储，甚至单个应用的功能，但有限的特定于用户的应用程序配置设置则可能是个例外。
Software as a Service (SaaS). The capability provided to the consumer is to use the provider’s applications running on a cloud infrastructure2. The applications are accessible from various client devices through either a thin client interface, such as a web browser (e.g., web-based email), or a program interface. The consumer does not manage or control the underlying cloud infrastructure including network, servers, operating systems, storage, or even individual application capabilities, with the possible exception of limited user-specific application configuration settings.
3.2、平台即服务（PaaS）
该模式的云服务，是将消费者创建或获取的应用程序，利用资源提供者指定的编程语言和工具部署到云的基础设施上。消费者不直接管理或控制包括网络，服务器，运行系统，存储，甚至单个应用的功能在内的底层云基础设施，但可以控制部署的应用程序，也有可能配置应用的托管环境。
Platform as a Service (PaaS). The capability provided to the consumer is to deploy onto the cloud infrastructure consumer-created or acquired applications created using programming languages, libraries, services, and tools supported by the provider.3 The consumer does not manage or control the underlying cloud infrastructure including network, servers, operating systems, or storage, but has control over the deployed applications and possibly configuration settings for the application-hosting environment.
3.3、基础设施即服务（IaaS）
该模式的云服务，是租用处理、存储、网络和其他基本的计算资源，消费者能够在上面部署和运行任意软件，包括操作系统和应用程序。消费者不管理或控制底层的云计算基础设施，但可以控制操作系统、存储、部署的应用，也有可能选择网络构件（例如，主机防火墙）。
Infrastructure as a Service (IaaS). The capability provided to the consumer is to provision processing, storage, networks, and other fundamental computing resources where the consumer is able to deploy and run arbitrary software, which can include operating systems and applications. The consumer does not manage or control the underlying cloud infrastructure but has control over operating systems, storage, and deployed applications; and possibly limited control of select networking components (e.g., host firewalls).
四、部署模型
4.1、私有云（Private cloud）
私有云是为一个用户/机构单独使用而构建的，可以由该用户/机构或第三方管理，存在预置（on premise）和外置（off premise）两个状态。
Private cloud. The cloud infrastructure is provisioned for exclusive use by a single organization comprising multiple consumers (e.g., business units). It may be owned, managed, and operated by the organization, a third party, or some combination of them, and it may exist on or off premises.
4.2、社区云（Community cloud）
社区云是指一些由有着共同利益（如，任务、安全需求、政策、遵约考虑等）并打算共享基础设施的组织共同创立的云，可以由该用户/机构或第三方管理，存在on premise或off premise两个状态。
Community cloud. The cloud infrastructure is provisioned for exclusive use by a specific community of consumers from organizations that have shared concerns (e.g., mission, security requirements, policy, and compliance considerations). It may be owned, managed, and operated by one or more of the organizations in the community, a third party, or some combination of them, and it may exist on or off premises.
4.3、公共云（Public cloud）
公共云对一般公众或一个大型的行业组织公开可用，由销售云服务的组织机构所有。
Public cloud. The cloud infrastructure is provisioned for open use by the general public. It may be owned, managed, and operated by a business, academic, or government organization, or some combination of them. It exists on the premises of the cloud provider.
4.4、混合云（Hybrid cloud）
混合云由两个或两个以上的云（私有云、社区云、或公共云）组成，它们各自独立，但通过标准化技术或专有技术绑定在一起，云之间实现了数据和应用程序的可移植性（例如，解决云之间负载均衡的云爆发（cloud bursting））。
Hybrid cloud. The cloud infrastructure is a composition of two or more distinct cloud infrastructures (private, community, or public) that remain unique entities, but are bound together by standardized or proprietary technology that enables data and application portability (e.g., cloud bursting for load balancing between clouds).