IaaS: Infrastructure-as-a-service (infrastructure as a service)
With IaaS, you can outsource your hardware to other places. IaaS will provide off-site servers, storage and network hardware that you can rent. Save on maintenance costs and office space, companies can use these hardware at any time to run their applications.
Some large IaaS companies include Amazon, Microsoft, VMWare, Rackspace and Red Hat. However, these companies have their own expertise, such as Amazon and Microsoft provide you with not only IaaS, they will also rent their computing power to you to host your site.
PaaS: Platform-as-a-service (Platform as a service)
The second layer is called PAAs, which is sometimes called middleware. All the development of your company can be done at this level, saving time and resources.
PAAs offers a variety of solutions for developing and distributing applications on the web, such as virtual servers and operating systems. This saves you money on the hardware and makes it easier to collaborate between decentralized studios. Web application management, application design, application virtual hosting, storage, security and application development collaboration tools, etc.
Some large PAAs providers have Google App engine,microsoft azure,force.com,heroku,engine Yard. The company that recently sprang up has appfog,mendix and standing Cloud.
SaaS: Software-as-a-service (software as a service)
The third layer is called SaaS. This layer is a layer of daily contact with your life, most of which is accessed through a Web browser. Any application on a remote server can be run over the network, which is SaaS.
The services you consume are completely accessible from Web pages such as Netflix,mog,google Apps,box.net,dropbox or Apple's icloud. Although these Web services are used for business and entertainment or both, this is part of cloud technology.
Some SaaS applications for business include Crm,adp,workday and SuccessFactors of Citrix's go to Meeting,cisco Webex,salesforce.
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PAAS, IaaS, and SaaS differences