What is Cloud Computing?

Let's start with a definition from Wikipedia:

"Cloud Computing is Internet-based computing, whereby shared resources, software and information are provided to computers and other devices on-demand, like a public utility."

Now I'll break it down into something a little easier to wrap your brain around:
Any computing service you use that doesn't live on a traditional computer server in your office is probably "Cloud Computing."

Many companies use an e-mail filtering service that removes spam and viruses before delivering that e-mail to employee inboxes. That's cloud computing.

Some companies now send a copy of their data backups to the internet for offsite storage. That's cloud computing.

A recent popular example of cloud computing is Salesforce.com, a customer relationship management (CRM) application you access through a web browser. You could also consider Facebook a cloud computing application.

There are a couple different levels of cloud computing:
1. Applications (Facebook, Salesforce, Hotmail, etc.)
2. Platforms (places for people to build applications and not put them on their own servers)
3. Infrastructure (replacing or augmenting your own servers or networking equipment with some in the cloud)

For the purpose of this discussion we'll really just concentrate on applications and infrastrcture. We'll let people who do computer programming worry about the platforms for now.