Several different flavors have sprung up in cloud computing and each has their pros and cons. Add to these the plethora of vendor-created acronyms and it can be confusing to figure out the best option.
The three primary types of cloud computing are IaaS, PaaS and SaaS - infrastructure, platform and software as a service, respectively. When you take a closer look, you'll see that what will decide this argument are your own company's needs and comfort level.
These services are made possible by virtualization, the ubiquity of high-speed networks and the capabilities of today's browsers. With these things in place, it becomes less necessary to own your own infrastructure, or even to own your own software. You can get what you need from the cloud, as you need it.
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The easiest way to understand these as-a-service offerings is to start with SaaS, the most abstract layer and the one you may already be using today, even at a personal level. A simple example of SaaS is an online email service, like Gmail. If you use Gmail, you are not hosting your own email server. Google is hosting it, and you are simply accessing it through your browser-as-client. But email is just one application that your company uses, and today there are applications online for many business purposes.
SaaS is really geared toward the end users in your organization and doesn't take much to get started. The provider figures out how many resources to devote to your use of the application. The provider figures out the servers, the virtual machines, the network equipment, everything. You just point your browser at it.
IaaS is at the other end of the cloud spectrum. In this scenario, you want to maintain control of your software environment, but you don't want to maintain any equipment. You don't want to have to buy servers and put them in a climate-controlled room or any of that. Instead, you go to an IaaS provider and request a virtual machine.
You can put whatever software you want on it. On the back end, the provider gets you storage or other resources as you need them. This is made easier with virtualization technologies, which separate the physical drives and so forth from the virtual machine you're running in. IaaS is available from Amazon EC2, IBM and many others, but care should be taken in choosing a provider.
PaaS is somewhere in between IaaS and SaaS. It's not a finished product, like SaaS, and it's not a tabula rasa, like IaaS. PaaS gives your application developers hooks and tools to develop to that particular platform. For example, Microsoft's Windows Azure gives you tools to develop mobile apps, social apps, websites, games and more. You build these things, but you use the APIs and tools to hook them into the Azure environment and run them there.
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Confusing the issue are marketers who have co-opted the "as-a-service" craze (just look at the many iterations of MaaS). Most of these are really types of SaaS, with variations on the "software" part.
In the end, SaaS and its many types may actually become so overused that we stop referring to it as a cloud thing. After all, even what we typically regard as, say, an e-commerce Web site is a kind of software-as-a-service, with bits of software running in the background that the user never sees except through the graphical user interface that is the Web page.
IaaS and PaaS will be the two broad classes of cloud computing, each appealing to a different set of customers with different technical skills. And once the industry becomes more comfortable with this whole cloud concept, the real argument in the future will be: How much of your computing do you trust to other providers, and how much do you keep in house?