云计算是一种向客户提供IT基础设施的方式，它不仅仅是一套要实施的产品。任何要成为云服务的服务，都需要满足以下五个标准——
- 按需自助服务：开始和停止服务的决定取决于客户，无需与供应商直接互动。
- 广泛的网络访问：服务必须对使用任何网络的任何设备可用。
- 资源池：提供商创建一个资源池并将其动态分配给客户。
- 快速弹性：提供商提供的服务必须易于扩展和快速。
- 衡量服务：提供商必须衡量服务的使用情况并相应地收取费用。跟踪使用情况也有助于改善服务。
1. 公有云：
服务提供商在互联网上公开所有资源的计算。它连接到公共互联网。服务提供商通过互联网向公众提供虚拟机、应用程序、存储等资源。它可能是免费的，也可能是按次付费。它可用于公开展示，谷歌使用云来运行它的一些应用程序，如谷歌文档、谷歌驱动器或 YouTube 等。
它是实现云计算的最常见方式。外部云服务提供商通过公共网络拥有、运营和交付它。
最适合需要基础设施以容纳大量客户并从事具有多元化组织(即研究机构和非政府组织等)的项目的公司。
2. 私有云：
服务提供商在互联网上公开所有资源的计算。它仅支持通过专用网络进行连接。它只有真实用户和单人架构。 Google Drive、Google docs 或 YouTube 等应用程序的 Google 后端数据不向公众开放，这些类型的数据和应用程序运行在私有云上。
基础设施和服务通过专用网络进行维护和部署；硬件和软件仅供私人公司使用，即特殊实体的成员。
由于它具有最佳的适应性和灵活性，因此最适合需要具有高性能、高安全性和隐私性的基础设施的公司。
下表列出了公共云和私有云之间的差异：
| Public Cloud | Private Cloud |
|---|---|
| Cloud Computing infrastructure shared to public by service provider over the internet. It supports multiple customers i.e, enterprises. | Cloud Computing infrastructure shared to private organisation by service provider over the internet. It supports one enterprise. |
| Multi-Tenancy i.e, Data of many enterprise are stored in shared environment but are isolated. Data is shared as per rule, permission and security. | Single Tenancy i.e, Data of single enterprise is stored. |
| Cloud service provider provides all the possible services and hardware as the user-base is world. Different people and organization may need different services and hardware. Services provided must be versatile. | Specific hardware and hardware as per need of enterprise are available in private cloud. |
| It is hosted at Service Provider site. | It is hosted at Service Provider site or enterprise. |
| It is connected to the public internet. | It only supports connectivity over the private network. |
| Scalability is very high, and reliability is moderate. | Scalability is limited, and reliability is very high. |
| Cloud service provider manages cloud and customers use them. | Managed and used by single enterprise. |
| It is cheaper than private cloud. | It is costlier than public cloud. |
| Security matters and dependent on service provider. | It gives high class of security. |
| Performance is low to medium. | Performance is high. |
| It has shared servers. | It has dedicated servers. |
| Example : Amazon web service (AWS) and Google AppEngine etc. | Example : Microsoft KVM, HP, Red Hat & VMWare etc. |