Reference - Azure

What is Cloud Computing?

Cloud computing is the delivery of computing services—including servers, storage, databases, networking, software, analytics, and intelligence—over the Internet (“the cloud”) to offer faster innovation, flexible resources, and economies of scale.

Types

Public

Azure, AWS, 阿里云。

Delivers all the computing resources, like servers and storage over the internet.

Private

Used exclusively by a single business or organization.

Could be located in a company’s datacenter.

Hybrid

Combine public and private clouds.

Types of Services: Iaas/PaaS/SaaS

Reference - 知乎

如果你想要建立一个网站。不采用云服务，你所需要的投入大概是：买服务器，安装服务器软件，编写网站程序。

现在你采用流行的云计算，

如果你采用IaaS服务，那么你就不用自己买服务器了，随便在哪家购买虚拟机，但是还是需要自己装服务器软件。

而如果你采用PaaS的服务，那么意味着你既不需要买服务器，也不需要自己装服务器软件，只需要自己开发网站程序。

如果你再进一步，购买某些在线论坛或者在线网店的服务,这意味着你也不用自己开发网站程序，只需要使用它们开发好的程序，而且他们会负责程序的升级、维护、增加服务器等，而你只需要专心运营即可，此即为SaaS。

IaaS

Rent IT infrastructure—servers and virtual machines (VMs), storage, networks, operating systems

Amazon EC2 / Azure / 阿里云 / Google Clouds

PaaS

Supply an on-demand environment for developing, testing, delivering, and managing software applications.

Developers can easily create web apps without worrying about setting up or managing the underlying infrastructure of servers / storages / networks/ database…

Heroku / Salesforce / Openshift

SaaS

Delivering software applications over the Internet, on demand and typically on a subscription basis.

Users connect to the application over the Internet, usually with a web browse on their phone, tablet, or PC.

Gmail / Paypal / QQ