In a previous post, “What The Heck Is Cloud Computing?” I broke the cloud up into three categories.  however I do not think I did a good job of delineating these categories with examples.  So here goes:

IaaS: Infrastructure as a Service  

• Storage: Amazon S3 (BLOB storage), Amazon SDB (Table Storage), Microsoft Windows Azure Simple Data Storage (both table and BLOB)
• Database: Microsoft SQL Azure, Amazon RDS
• Virtual servers: Amazon EC2

PaaS: Platform as a Service

• Microsoft Windows Azure Fabric Services, Google App Engine

SaaS: Software as a Service

• Pretty much every application accessed through a website.  Pick your favorite webmail or photo hosting site for instance.
• But some stand out as more fully featured applications run from the browser more than as mere websites: Microsoft Outlook Web Access, Google Docs, Microsoft Office Web Apps.

And thanks to a heads-up from Dare Obasanjo, I found a somewhat different labeling of the cloud from Tim O'Reilly in his blog post Web 2.0 and Cloud Computing.  His breakdown is:

Utility computing

• “…virtual machine instances, storage, and computation at pay-as-you-go utility pricing…”

Platform as a Service

• “…which hide machine instances behind higher-level APIs”

Cloud-based end-user applications

• “Any web application is a cloud application in the sense that it resides in the cloud. Google, Amazon, Facebook, twitter, flickr, and virtually every other Web 2.0 application is a cloud application in this sense. However, it seems to me that people use the term "cloud" more specifically in describing web applications that were formerly delivered locally on a PC, like spreadsheets, word processing, databases, and even email”

where these labels pretty much correspond 1 to 1 to the ones above.

I hope this clarifies things a bit.

This is part 2 of a series:

1. What The Heck Is Cloud Computing?
2. <this post>
3. What The Heck Is Cloud Computing? – Another re-look, with Pretty Pictures