The cloud computing services offered by the providers fall into three broad categories:

• Software as a Service (SaaS)
• Platform as a Service (PaaS)
• Infrastructure as a Service (IaaS)

This classification led to the definition of a scheme that takes the name of the SPI model (see the bold initials in the previous list). Sometimes it is called the cloud computing stack, as these categories are based on each other.

A detailed description of each of these levels will now be given, following a top-down approach.