Cloud computing is Internet- ("cloud-") based development and use of computer technology ("computing").[1] In concept, it is a paradigm shift whereby details are abstracted from the users who no longer need knowledge of, expertise in, or control over the technology infrastructure "in the cloud" that supports them.[2] Cloud computing describes a new supplement, consumption and delivery model for IT services based on Internet, and it typically involves the provision of dynamically scalable and often virtualized resources as a service over the Internet.[3][4]
以上内容来自Wikipedia，http://en.wikipedia.org/wiki/Cloud_computing
 
  1. Cloud 的layer主要包括以下几个：client、application、platform、 infrastructure、servers。
  2. cloud的types主要分为以下几个：public cloud（such as amazon、google、microsoft‘s azure）、hybrid cloud、private cloud。
  3. cloud的服务类型主要分为以下几个：Infrastructure-as-a-Service (IaaS，such as  Amazon Web Services )、Platform-as-a-Service (PaaS，such as Google Apps)、Software-as-a-Service (SaaS，such as twitter)、Cloud Service Level Agreements (SLAS)。
  4. cloud的主要特点是：Virtualization and automation、Interchangeable (fungible) resources such as servers, storage and network、Management of these resources as a single fabric、Elastic capacity (scale up or down) to respond to business demands、Applications (and the tools to develop them) that can truly scale out、Focused on the service delivered to the business。