What is the difference between Cloud and SaaS?

Very simply put SaaS is a subset of cloud enabled services. With SaaS we usually also discuss the IaaS and PaaS service models and the differences between the three. Furthermore, cloud solutions have also evolved where they can be private, hybrid or public. NIST has several whitepapers that very clearly articulate these differences but wikipedia has good start as well:
http://en.wikipedia.org/wiki/Cloud_computing

"Cloud" and "SaaS" are two separate, but overlapping concepts. Let's take them one at a time, and then discuss their interaction.

"Cloud" in it's simplest form refers to shared resources - data, software applications,
storage, communications capabilities, and computer processing power. (For details, see my video, "What Is Cloud Computing?" at http://www.e-com.com/sample-client-videos.) The Public Cloud is what most people think of when they refer to "The Cloud." These are resources accessible to many people and organizations through the public Internet.

There are also, Private, Hosted Private, and Hybrid Clouds. (See my paper, "Cloud Computing – Public, Private, and Hybrid," at http://www.e-com.com/white-papers-and-blogs#DynaSis, for a more detailed discussion of types of clouds.)

A Private Cloud is one whose resources are all accessed and controlled by a single entity (e.g., a company or a public sector organization). Larger enterprises may create their own private clouds and host them on infrastructure which they also own. This can allow multiple divisions, departments, etc., to share resources more cost effectively. Smaller organizations may purchase services from a third-party Cloud Services Provider (CSP) to host their Private Cloud for them. Often the resources in this Hosted Private Cloud are integrated with internally hosted resources. The result is what's often called a Hybrid Cloud.

Software-as-a-Service, or SaaS, is really a software licensing concept. It means that people or organizations can use software on a per user, per transaction, or "per" some other easily measurable variable basis rather than purchasing the software outright and installing it on their own internal infrastructure.

To be economically viable, most SaaS vendors provide access to their software through the Public Cloud. In theory, however, SaaS could be hosted in a Private Cloud, though this is rarely done.

At the same time, SaaS is just one of multiple kinds of services available through Cloud Computing. These include Infrastructure as a Service, or IaaS (available through both Public and Hosted Private CSP's), Platform as a Service, or PaaS, Cloud-based Storage, and even Cloud Based Communications.

John, it may help to understand to origin of the term "cloud". Early papers describing networking and the Internet initially used the term as a broad reference to everything beyond the desktop/notebook PC . More recently it has been combined in the expression "Cloud Computing" to reference the various concepts already described by Jack Shaw and others.
In general today, it is used as a reference to using hardware and software resources available via the Internet. Using Google search, YouTube, Facebook, Focus etc. are simple examples of using cloud computing.
Cloud Computing references a broad concept of using resources beyond the desktop (and more often today beyond the enterprise) while SaaS, etc are business and delivery models.

'Cloud' is an environment, wheras 'SaaS is a business model.

At present, we can view the SaaS as one of the delivery models of a cloud environment where the intended consumer are the direct users of the application. Other delivery models that are residing on the cloud environment are P(latform) aaS and I(infrastructure) aaS which target different of user group. PaaS is normally for developers while IaaS is for systems administrator.

I'm not really clear about the difference. I hope that reading the many answers to this question
will give me a clearer insight. Looking forward to reading all of your input. Thanks!

The term cloud is used as a metaphor for the Internet, based on the cloud drawing used in the past to represent the telephone network, and later to depict the Internet in computer network diagrams as an abstraction of the underlying infrastructure it represents. Cloud computing is a model for enabling ubiquitous, convenient, on-demand network access to a shared pool of configurable computing resources that can be rapidly provisioned and released with minimal management effort or service provider interaction. This cloud model is composed of five essential characteristics, viz., on-demand self-service; broad network access; resource pooling; rapid elasticity; and measured service.
The cloud model is composed of three service models, one of which is Software as a Service (SaaS). In Software as a Service (SaaS) service model, the capability provided to the consumer is to use the provider’s applications running on a cloud infrastructure. The applications are accessible from various client devices through either a thin client interface, such as a web browser (e.g., web-based email), or a program interface. The consumer does not manage or control the underlying cloud infrastructure including network, servers, operating systems, storage, or even individual application capabilities, with the possible exception of limited user-specific application configuration settings.
For more information, click here –
http://www.myrealdata.com/cloud-computing.html

What's the difference between beer and Lager ? SaaS is merely one of the three canonical Service Models, which also include "Infrastructure" and "Platform". SaaS provides the most fully baked IT solution with Platform and Infrastructure descending in business value, but ascending in flexibility.

If you Google "NIST cloud definition" you will come upon a simple two-page definition of Cloud from an organization with no agenda other than to educate the public. We should be way beyond asking about cloud definitions at this point in time.

My understanding of SaaS is what several users have mentioned, that you're merely "subscribing" to the software, analogous to renting an apartment vs. buying a condo. But I think the term "cloud computing" is more broadly used to refer to the file-sharing and backup solutions in addition to the rented software.

And my concern with using cloud computing as one's primary storage or software solution is what happens to your data if you decide you no longer want to subscribe to that service, or if something goes wrong with your payment and they cut off your access to your files. I've heard too many stories of people who'd hired a website designer, then had that person hold their URL hostage when they wanted to switch services, to ever leave control of my data in somebody else's hands. Cloud solutions may be great for backups, but I always want a copy of my stuff on my own server!

Some would say they are synonymous, while some will say that SaaS is not cloud, but might use cloud to deliver a service to it's clients. SaaS has been included in the definition of cloud computing because the early adopters were looking for on-demand, measurable ways to acquire IT services. This initially broke down across 3 boundaries: pure infrastructure, templated platforms and complete applications. These boundaries are now starting to blur and change, which means that the current definitions will become albatrosses around the cloud's neck instead of a useful means of discussing the cloud computing pattern.

-JPM
http://about.me/jpmorgenthal