While not as confusing as the web 2.0 term, cloud computing has been defined in many different ways and everybody felt like they should contribute their own version.
The National Institute of Standards and Technology is proposing some definitions that are better than many others I have read so far:
Cloud computing is a model for enabling convenient, on-demand network access to a shared pool of configurable computing resources (e.g., networks, servers, storage, applications, and services) that can be rapidly provisioned and released with minimal management effort or service provider interaction.
The document also details on the 5 main characteristics mentioned in the definition:
The definition would not be complete without also talking about the different service models:
and also the different deployment models:
I would strongly encourage you to download the document and read it.