What is the definition of cloud computing is?

In the top level, cloud computing can be divided into: IaaS, PaaS, SaaS. IaaS refers to the physical state of the computing, storage, network resources abstract state calculation software form, storage, network resources, unified management, and supplied on demand to provide an elastic model, linked data is to represent the virtual machine. PaaS refers middleware, database software and other platforms; SaaS refers to the upper application software, such as collaboration software, SaaS, SaaS financial software.
Cloud computing by type is divided into: public cloud, industry cloud, private cloud, hybrid cloud, which telecommunications companies to provide all cloud services, private cloud is the cloud platform to deploy in their own data center, only to their own use, hybrid cloud can is a public private cloud and hybrid cloud, it can be mixed and private cloud industry, mainly security reasons, all interconnected customer data is not important, non-core, non-classified businesses into the public cloud / cloud industry, will be important the core business classified into the private cloud.