Clouds?

Generally, there are three types of clouds: Software-as-a-Service (Saas), Platform-as-a-Service (PaaS), and Infrastructure-as-a-Service (IaaS).

Software-as-a-Service (SaaS) is the same model that’s been around for years but has lately taken advantage of the cloud moniker. Google Apps and Salesforce.com are the big SaaS players but there are SaaS offerings for all kinds of business applications, including content management.

The allure of SaaS is in general:

• Lower up-front costs
• Someone else gets to worry about running and scaling the infrastructure

The challenges of SaaS include things like:

• The ability to do heavy customization and complex workflows
• Ease of integration with other systems
• Client perceptions (and real issues) around data security
• Data portability/vendor lock-in

The next type of cloud is Platform-as-a-Service (PaaS). The two best examples of PaaS are Google App Engine (GAE) and Salesforce.com’s force.com platform. With PaaS, you provide the code and the PaaS provider does the rest. Of course this means your code has to follow certain standards and is often subject tolimitations, but the beauty is that you get a completely custom solution without worrying about any of the infrastructure.

For certain applications, the benefits of instantaneous, global scale far outweigh the limitations of the platform. For example, I’ve seen apps in the Salesforce.com AppExchange that are basically tools for building a web site that’s tightly integrated with Salesforce.com. I think you’ll also see solutions that leverage a PaaS for certain components or sub-systems.

The third type of cloud is Infrastructure-as-a-Service (IaaS). An IaaS cloud is about providing virtual servers on-demand. Examples include things like Amazon’s EC2, Rackspace Cloud, and GoGrid. With these services you can instantly provision as many servers as you need. What you do with them is up to you. When you’re done, you turn them off. Specifics vary but you are essentially billed for CPU time.

Source : http://ecmarchitect.com/archives/2009/07/08/1014