Infrastructure as a service (IaaS) cloud generally implies a few distinguishing features such as hourly utility billing and no long term commitments, rapid elasticity (provision/de-provision in minutes, not hours) and usually (but not always - there are bare metal clouds) virtualization. People often refer to software as a service (SaaS) as cloud, which I think is where it starts getting very ambiguous (i.e. is any hosted web application in the cloud???)