"Cloud" has become a very over-hyped term over the last few years. One of the characteristics of over-hyped words is that they have little or no definition to them (yes NosqlDefinition I'm looking at you).
As it turns out there is an excellent definition of cloud computing available, from none other that NIST. It's available by a wonderfully short and easy to understand standards document (no, I'm not kidding).
NIST defines cloud as access to a pool of computing resources (servers, storage, networks, services etc) that can be rapidly provisioned and made available with minimal overhead.
It briefly defines
- five essential characteristics: on demand self-service, broad network access, resource pooling, rapid elasticity, and measured service.
- three service models: software, platform, and infrastructure (all as a service).
- four deployment models: private, community, public, and hybrid.