clipped from www.infoworld.com

1. SaaS
This type of cloud computing delivers a single application through the browser to thousands of customers using a multitenant architecture. On the customer side, it means no upfront investment in servers or software licensing; on the provider side, with just one app to maintain, costs are low compared to conventional hosting. Salesforce.com is by far the best-known example among enterprise applications, but SaaS is also common for HR apps and has even worked its way up the food chain to ERP, with players such as Workday. And who could have predicted the sudden rise of SaaS “desktop” applications, such as Google Apps and Zoho Office?

2. Utility computing
The idea is not new, but this form of cloud computing is getting new life from Amazon.com, Sun, IBM, and others who now offer storage and virtual servers that IT can access on demand. Early enterprise adopters mainly use utility computing for supplemental, non-mission-critical needs, but one day, they may replace parts of the datacenter. Other providers offer solutions that help IT create virtual datacenters from commodity servers, such as 3Tera’s AppLogic and Cohesive Flexible Technologies’ Elastic Server on Demand. Liquid Computing’s LiquidQ offers similar capabilities, enabling IT to stitch together memory, I/O, storage, and computational capacity as a virtualized resource pool available over the network.

3. Web services in the cloud
Closely related to SaaS, Web service providers offer APIs that enable developers to exploit functionality over the Internet, rather than delivering full-blown applications. They range from providers offering discrete business services — such as Strike Iron and Xignite — to the full range of APIs offered by Google Maps, ADP payroll processing, the U.S. Postal Service, Bloomberg, and even conventional credit card processing services.

4. Platform as a service
Another SaaS variation, this form of cloud computing delivers development environments as a service. You build your own applications that run on the provider’s infrastructure and are delivered to your users via the Internet from the provider’s servers. Like Legos, these services are constrained by the vendor’s design and capabilities, so you don’t get complete freedom, but you do get predictability and pre-integration. Prime examples include Salesforce.com’s Force.com, Coghead and the new Google App Engine. For extremely lightweight development, cloud-based mashup platforms abound, such as Yahoo Pipes or Dapper.net.

5. MSP (managed service providers)
One of the oldest forms of cloud computing, a managed service is basically an application exposed to IT rather than to end-users, such as a virus scanning service for e-mail or an application monitoring service (which Mercury, among others, provides). Managed security services delivered by SecureWorks, IBM, and Verizon fall into this category, as do such cloud-based anti-spam services as Postini, recently acquired by Google. Other offerings include desktop management services, such as those offered by CenterBeam or Everdream.

6. Service commerce platforms
A hybrid of SaaS and MSP, this cloud computing service offers a service hub that users interact with. They’re most common in trading environments, such as expense management systems that allow users to order travel or secretarial services from a common platform that then coordinates the service delivery and pricing within the specifications set by the user. Think of it as an automated service bureau. Well-known examples include Rearden Commerce and Ariba.

clipped from blogs.msdn.com

Software as a Service (SaaS) – uses a multi-tenant architecture to deliver a single application through the browser to thousands of customers, offering savings in infrastructure investment or software licensing.

Utility computing – offers storage and virtual servers that can be accessed on demand and are used mainly for supplemental, non-mission-critical needs.

Web services in the cloud – offers APIs that enable developers to exploit functionality over the Internet, rather than delivering full-blown applications.

Platform as a service (PaaS) — delivers development environments as a service, allowing you to build your own applications that run on the provider’s infrastructure and are delivered to your users via the Internet from the provider’s servers.

Managed Service Providers (MSP) – offers an environment in which an application is exposed to IT rather than to end-users.

Service commerce platforms – are a hybrid of SaaS and MSP and offer a service hub that users interact with.

A seventh option is Microsoft’s Software-plus-services (S+S), which is designed to bring together the best of cloud-based, hosted services and the software that resides on a variety of devices to provide flexible and effective solutions. This model is based on the premise that information workers need flexibility in the way in which they access services and IT departments need the flexibility to decide which workloads they are most comfortable moving to the cloud and which are better left on-premise.  Depending on your IT model and staffing and agency needs, you can choose from three S+S delivery models or create a hybrid of any of the following:

On-premise – host software at your location, on your servers.

Partner-hosted – secure a Microsoft partner to host your agency applications.

Microsoft-hosted – choose a Microsoft-hosted service.