The various types of Cloud offerings

There are three main types of Cloud offerings, these are:

• Infrastructure-as-a-Service (IaaS) provides basic compute and storage resources. Vendors in this space include Amazon (with it's EC2 product).
• Platform-as-a-Service (PaaS) provides an application services framework. Vendors in this space include Microsoft (with Azure), Google (with Google Apps Engine) and Salesforce (with Force.com).
• Software-as-a-Service (SaaS) provides complete applications. Vendors in this space include Microsoft (with Office 365), Google (with GMail/Google Docs/Google Apps) and SalesForce (with Salesforce.com).

Each offers an increasing level of abstraction, with SaaS, being the highest level of abstraction, delivering shrink wrapped products with little room for customisation.

Note the distinctions between the three, each is a very different offering with very distinct characteristics. Matching your requirements against the type of offering allows you to narrow down the list of appropriate vendors dramatically.

Notice also the delineation between the IaaS and PaaS/SaaS vendors. The PaaS vendors generally also have SaaS offerings (using their platform to deliver software) but there's no major vendor with offerings that cross over from IaaS to PaaS/SaaS.