因为这个问题有很多更好的定义。这里我想再举几个例子:

IaaS

AWS EC2 Rackspace 谷歌计算引擎(GCE) 数字海洋 Magento 1企业版 甲骨文云 Vultr

PaaS

AWS弹性豆茎 Heroku Windows Azure Force.com OpenShift Apache Stratos Magento商业云

SaaS

谷歌应用程序 Salesforce Dropbox MailChimp的 ZenDesk DocuSign 松弛 Hubspot的 思科 网讯 同意 进行交流

IaaS、PaaS和SaaS基本上都是云计算领域。

IaaS(基础设施即服务)——基础设施即服务是云计算的一种提供模型，在这种模型中，组织将用于支持运营的设备外包，包括存储、硬件、服务器和网络组件。服务提供商拥有设备，并负责设备的安装、运行和维护。客户通常按使用次数付费。前亚马逊网络服务，BlueLock, Cloudscaling和Datapipe

PaaS (Platform as a Service) - Platform as a Service is one of the GROWING sector of cloud computing. PaaS basically help developer to speed the development of app, saving money and most important innovating their applications and business instead of setting up configurations and managing things like servers and databases. In one line I can say Platform as a service (PaaS) automates the configuration, deployment and ongoing management of applications in the cloud. Ex: Heroku, EngineYard, App42 PaaS and OpenShift

SaaS(软件即服务)——软件即服务，SaaS是一种软件交付方法，它以基于web的服务的形式提供对软件及其功能的远程访问。阿比柯和阿卡迈

SaaS: Software as a Service Cloud application services or “Software as a Service” (SaaS) are probably the most popular form of cloud computing and are easy to use. SaaS uses the Web to deliver applications that are managed by a third-party vendor and whose interface is accessed on the clients’ side. Most SaaS applications can be run directly from a Web browser, without any downloads or installations required. SaaS eliminates the need to install and run applications on individual computers. With SaaS, it’s easy for enterprises to streamline their maintenance and support, because everything can be managed by vendors: applications, runtime, data, middleware, O/S, virtualization, servers, storage, and networking. Gmail is one famous example of an SaaS mail provider.

PaaS: Platform as a Service The most complex of the three, cloud platform services or “Platform as a Service” (PaaS) deliver computational resources through a platform. What developers gain with PaaS is a framework they can build upon to develop or customize applications. PaaS makes the development, testing, and deployment of applications quick, simple, and cost-effective, eliminating the need to buy the underlying layers of hardware and software. One comparison between SaaS vs. PaaS has to do with what aspects must be managed by users, rather than providers: With PaaS, vendors still manage runtime, middleware, O/S, virtualization, servers, storage, and networking, but users manage applications and data.

IaaS: Infrastructure as a Service Cloud infrastructure services, known as “Infrastructure as a Service” (IaaS), deliver computer infrastructure (such as a platform virtualization environment), storage, and networking. Instead of having to purchase software, servers, or network equipment, users can buy these as a fully outsourced service that is usually billed according to the amount of resources consumed. Basically, in exchange for a rental fee, a third party allows you to install a virtual server on their IT infrastructure. Compared to SaaS and PaaS, IaaS users are responsible for managing more: applications, data, runtime, middleware, and O/S. Vendors still manage virtualization, servers, hard drives, storage, and networking. What users gain with IaaS is infrastructure on top of which they can install any required platforms. Users are responsible for updating these if new versions are released.

IaaS(基础设施即服务)

IaaS提供基础设施，如虚拟机和其他资源，如虚拟机磁盘映像库、基于块和文件的存储、防火墙、负载平衡器、IP地址、虚拟局域网等。基础设施即服务(IaaS)是云计算模型的基础层。

常见的例子:DigitalOcean、Linode、Rackspace、Amazon Web Services (AWS)、Cisco Metapod、Microsoft Azure、谷歌Compute Engine (GCE)是Iaas的一些流行例子。

平台即服务(PaaS)

PaaS或平台即服务模型为您提供计算平台，通常包括操作系统、编程语言执行环境、数据库和web服务器。从技术上讲，它是在IaaS之上的一个层，是继基础设施之后你需要的第二个东西，是一个平台。

常见的例子:AWS Elastic Beanstalk, Windows Azure, Heroku, Force.com，谷歌应用程序引擎，Apache Stratos。

软件即服务(SaaS)

在SaaS中，您可以访问安装在服务器上的应用程序服务。你不必担心软件的安装、维护或编码。您只需使用浏览器即可访问和操作该软件。你不需要下载或安装任何类型的安装或操作系统，软件只是供你访问和操作。软件维护或设置或帮助将由SaaS提供商公司提供，您只需支付使用费用。

常见的例子:谷歌应用程序，微软office365，谷歌文档，Gmail, WHMCS计费软件

IaaS, PaaS和SaaS之间的基本区别

对于傻瓜:

IAAS(基础设施即服务):

基础层 处理虚拟机，存储(硬盘)，服务器，网络，负载均衡器等

PAAS(平台即服务):

IAAS之上的一层 运行时(如java运行时)，数据库(如mySql, Oracle)， Web服务器(tomcat等)

软件即服务(SAAS):

PAAS上的一个层 应用程序，如电子邮件(Gmail，雅虎邮件等)，社交网站(Facebook等)

为了快速关联，考虑以下谷歌的产品:

IAAS: 谷歌计算引擎(可以开发在高性能谷歌计算基础设施上运行的程序)

PAAS: 谷歌应用程序引擎(一个可以开发应用程序，让他们在谷歌应用程序引擎上执行，它负责执行)

SAAS: Gmail，谷歌+等(可以使用电子邮件服务，并扩展基于电子邮件/谷歌+的应用程序，以形成更新的应用程序)

受欢迎程度

公司明智的知名度

云计算是由

亚马逊网络服务(AWS) 谷歌计算引擎，谷歌应用引擎 微软Azure 有许多中小型云运营商，包括IBM、甲骨文等。

这些服务的受欢迎程度主要归功于公司的声誉以及这些公司在云空间方面的投资。

服务类型明智的知名度

PAAS (Platform as a Service) is more popular among developers as they can put all their concentration on developing their apps and leave the rest of management and execution to the service provider. Many service providers also offer the flexibility to increase/decrease the CPU power depending upon the traffic loads giving developers cost effective and easy & effortless management. SAAS (Software as a service) is more popular among with consumers, who bother about using the application such as email, social networking etc IAAS (Infrastructure as a service) is more popular among users into research and high computing areas.

如果你是一个简单的客户，想要使用某个软件，但手头什么也没有，那么你就使用SaaS。

当你有一个自己开发的软件，但你想在一个公共平台上部署和运行，那么你就使用PaaS。

当你已经准备好了软件和平台，但还想让硬件运行时，你就可以使用IaaS。