Service ModelsCloud computing providers offer their services according to three fundamental models:[3][33]Infrastructure as a service (IaaS), platform as a service (PaaS), and software as a service (SaaS) where IaaS is the most basic and each higher model abstracts from the details of the lower models.

[edit]Infrastructure as a service (IaaS)See also:Category:Cloud infrastructureIn this most basic cloud service model, cloud providers offer computers as physical or more often asvirtual machines, raw (block) storage,firewalls,load balancers, and networks. IaaS providers supply these resources on demand from their large pools installed indata centers.Local area networksincluding IP addresses are part of the offer. For thewide areaconnectivity, the Internet can be used or - incarrier clouds- dedicatedvirtual private networkscan be configured.To deploy their applications, cloud users then install operating system images on the machines as well as their application software. In this model, it is the cloud user who is responsible for patching and maintaining the operating systems and application software. Cloud providers typically bill IaaS services on autility computingbasis, that is, cost will reflect the amount of resources allocated and consumed.Infrastructure-as-a-Service orIaaS Cloudis a platform through which businesses can avail equipment in the form of hardware, servers, storage space etc. at pay-per-use service.Moreover, IaaS is a branch of cloud computing that has gathered attention among the entrepreneurs largely with the prime motive to make their business environments more organized and in sync with the ongoing operational activities of organizations.When we talk about IaaS functioning, it is not a machine that we are talking about, which does all the work, it is simply a facility given to the business enterprises that offers users the leverage of extra storage space in servers and data centers.Examples of IaaS include: Amazon CloudFormation (and underlying services such as EC2), Rackspace Cloud, and RightScales.[edit]Platform as a service (PaaS)Main article:Platform as a serviceSee also:Category:Cloud platformsIn thePaaSmodel, cloud providers deliver acomputing platformand/orsolution stacktypically including operating system, programming language execution environment, database, and web server. Application developers can develop and run their software solutions on a cloud platform without the cost and complexity of buying and managing the underlying hardware and software layers. With some PaaS offers, the underlying compute and storage resources scale automatically to match application demand such that cloud user does not have to allocate resources manually.Examples of PaaS include:Amazon Elastic Beanstalk,Heroku,EngineYard,Google App Engine, andMicrosoft Azure.[edit]Software as a service (SaaS)Main article:Software as a serviceIn this model, cloud providers install and operateapplication softwarein the cloud and cloud users access the software fromcloud clients. The cloud users do not manage the cloud infrastructure and platform on which the application is running. This eliminates the need to install and run the application on the cloud user's own computers simplifying maintenance and support. What makes a cloud application different from other applications is itselasticity. This can be achieved by cloning tasks onto multiplevirtual machinesat run-time to meet the changing work demand.[34]Load balancersdistribute the work over the set of virtual machines. This process is inconspicuous to the cloud user who sees only a single access point. To accommodate a large number of cloud users, cloud applications can bemultitenant, that is, any machine serves more than one cloud user organization. It is common to refer to special types of cloud based application software with a similar naming convention:desktop as a service, business process as a service,Test Environment as a Service,communication as a service.The pricing model for SaaS applications is typically a monthly or yearly flat fee per user.[35]Examples of SaaS include: Google Apps, Quickbooks Online and Salesforce.com.[edit]Cloud clientsSee also:Category:Cloud clientsUsers access cloud computing using networked client devices, such asdesktop computers,laptops,tabletsandsmartphones. Some of these devices -cloud clients- rely on cloud computing for all or a majority of their applications so as to be essentially useless without it. Examples arethin clientsand the browser-basedChromebook. Many cloud applications do not require specific software on the client and instead use aweb browserto interact with the cloud application. WithAjaxandHTML5theseWeb user interfacescan achieve a similar or even betterlook and feelas native applications. Some cloud applications, however, support specific client software dedicated to these applications (e.g.,virtual desktopclients and most email clients). Some legacy applications (line of business applications that until now have been prevalent in thin client Windows computing) are delivered via a screen-sharing technology.