A brief taxonomy of the cloud computing universe

Let's start with (yet another) taxonomy of SaaS, IaaS et al. to make clear what we mean by what.
The following illustration compares the "Pre-Cloud IT-Universe" to its a little more cloud companion universe.

Fig. 1: Cloud Taxonomy

On the highest level, the Presentation Layer to revive the old OSI terminology for a moment the cloud universe places the browser, as the universal user interface. While the browser appears to replace an enormous multitude of user interfaces in this universe, the actual change from a user perspective is minimal as he look and feel of the "old" world is usually preserved.
The same is true for the application. Here, just the method of delivery is changed: from physical installation on the users hard drive to provisioning of digital functionality on demand through the network. This is our definition of Software-as-a-Service or SaaS.

Since most of the actual computation is done on a remote machine, the performance requirement for the client systems is dramatically reduced. In the extreme case, when no software is installed on the client system at all, the hard-drive itself becomes superfluous and a simple so called Thin Client is sufficient. Software-wise such systems only posses a minimal Operating System, because the actual Windows or Linux is also running on a server somewhere in the network.
Such minimal PC-systems usually consist of a keyboard, a mouse, a graphics engine and a microprocessor. Current thin client hardware has become so small that it fits into the monitor or the wall plug. This basically describes our view of Desktop-as-a-Service or DaaS.

The term Platform-as-a-Service or PaaS is a tad more tricky to define. PaaS is often used to describe the middleware offered by a hosting provider which enables software vendors to integrate their applications into the providers SaaS environment. It also provides the framework to customise software instances to the users requirements which is especially important for enterprise users. Furthermore this platform has to offer means for data storage, retrieval, description and classification in order to use a data base across multiple applications. It may also contain (business) process related functionality.

Infrastructure-as-a-Service of IaaS simply describes the provision of CPU-power on demand. Here virtual servers, created and released when needed by the user, are provided as standardised virtual server instances on a scalable, high performance computing platform. In effect here the relatively young concept of a dedicated server is once more replaced by the ancient mainframe paradigm. Utility computing and cloud computing are broader terms for the same concepts.

We furthermore define ITaaS or IT-as-a-Server to include IaaS, DaaS, and SaaS as well as storage virtualisation.

Of course there are many definitions, classifications and taxonomy out there. We just happen to like the above taxonomy.