It’s a rehash term for web based applications. A famous and the most popular cloud computing is a web based email service like Gmail or Hotmail etc.. Where all the work and storage is done online as in the clouds aka datacenter rather than on your PC. Since most computing and all the storage is done in the clouds. One can email using a mobile phone or tablet or PC. Making access ubiquitous.
Microsoft’s Outlook/Office would be a good example of what isn’t a cloud service where most of your work is done on your PC rather than the cloud. Also including in not a cloud service would be Adobe Photoshop etc…
Nowadays most what one does is pretty much in the clouds. If you do Facebook, Twitter, Gmail and Google Docs then you’ve already been doing cloud computing.
Best way to tell is if you turn off your computer, are you going to lose the ability to retrieve what you’ve been working on? If no, then you’ve been working in the clouds.