Software As A Service (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.
Utility Computing – Now being made more popular by Amazon, Sun and IBM, it offers 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.
Web Services in the Cloud – 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, to the full range of APIs offered by Google Maps and ADP payroll processing.
Platform As A Service (PaaS) – An 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.
Managed Service Providers (MSPs) – 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. 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.
Service Commerce Platforms – 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.
Definitions from InfoWorld article.