Cloud as a service (give it data and a process and you get results) is one definition of cloud computing. The other is the architecture itself (large scale data management). In order to have cloud as a service, you need the architecture. And that's the biggest problem we have today. Designing a highly scalable architecture is not easy. And without this architecture (which is software based), some other company cannot just create their own cloud (which beats the security issues with cloud as a service). So once a good distributed software system (hadoop, open neptune, etc.) is 'perfected', this will be more possible.

And that should answer your question - the actual architecture itself for cloud computing, is the software which every machine in the cloud runs.