For a short and quick answer, cloud computing is moving your computer experience from your desktop to the internet (the cloud!).
From the consumer of computing services perspective, it means having those services accessible from the internet:
- Online application (e.g. Google Mail, Google Docs, etc.) ;
- Online storage (e.g. S3Fox + Amazon S3, Flickr, Apple MobileMe, etc.) ;
- Online services (e.g. Google Maps, Airline reservation, etc.).
From an IT person or software developer perspective, it means that you deploy your application on a (potential virtual) infrastructure or platform on the internet:
- Virtual infrastructure or grid (e.g. Sun Grid, Amazon EC2, though both have their own specificities) ;
- Platform (e.g. Google Apps Engine) ;
- Identity verification, storage, payments, etc.