There are three types of cloud hosting services.
- Infrastructure as a Services (IaaS) (Find ERP Cloud Hosting Providers)
- Platform as a Service (Paas) (Find ERP Cloud Hosting Providers)
- Software as a Service (SaaS) (Find ERP Cloud Hosting Providers)
Infrastructure as a service
In this most basic cloud service model, cloud providers offer computers – as physical or more often as virtual machines –, raw (block) storage, firewalls, load balancers, and networks. IaaS providers supply these resources on demand from their large pools installed in data centers. Local area networks including IP addresses are part of the offer. For wide area connectivity, the Internet can be used or -- in carrier clouds -- dedicated virtual private networks can 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 a utility computing basis, that is, cost will reflect the amount of resources allocated and consumed.
IaaS refers not to a machine that does all the work, but simply to a facility given to businesses 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, Google Compute Engine, and RightScale.
Platform as a service (PaaS)
In the PaaS model, cloud providers deliver a computing platform typically 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, and Microsoft Azure.
Software as a service (Saas)
In this model, cloud providers install and operate application software in the cloud and cloud users access the software from cloud 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 its elasticity. This can be achieved by cloning tasks onto multiple virtual machines at run-time to meet the changing work demand. Load balancers distribute 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 be multitenant, 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.
Examples of SaaS include: Google Apps, Quickbooks Online and Salesforce.com.
Text for the above is available at http://en.wikipedia.org/wiki/cloud_computing