What is exactly a platform as a service? [closed]

Question

My question is what exactly is platform as a service, and if my concepts of IaaS and SaaS are really correct.

IaaS: When you can specify your needs at machine level you're consuming IaaS, it means that when you're able to decide if your application, hosted on the cloud, runs within a 1.7GHz box or a 2.5Ghz one. Or even how many boxes you need.

SaaS: When you consume a software hosted on the internet, at user level. So if you use Google Docs youre using SaaS.

I cant clear undestand what is platform as a service. My doubt arises from when I read something like this:

Cloud computing has evolved to include platforms for building and running custom applications, a concept known as “platform as a service” (or PaaS). PaaS applications are also referred to as on-demand, Web-based, or software as a service (or SaaS) solutions. [http://www.salesforce.com/paas/]

Does it means that PaaS and SaaS are the same? What I could undestand by now, and Im not sure about it is that PaaS is when you rent, for example, a database on the cloud, say, SQL Azure... Youre renting something that isn't directly infrastructure nor software, but something that helps you to develop your final SaaS product. Am I right???

Answer 1

No, they're not the same thing. Say you wanted to implement your own version of SaaS like Google Docs. But say you don't want to provide the hardware and software necessary to implement the distributed storage, backup, and so on. If I provided that to you for a fee, that would be PaaS.

An oversimplified version of the difference is this: The software provides services to its users (spreadsheet, salesforce management). The platform provides services to the software (cloud storage, distributed database, backup). The infrastructure provides services to the platform (memory, local storage, CPU, network).