A website can be hosted on a physical box or *cloud*. But that doesn't mean that cloud computing and web hosting are the same.
Cloud computing is the delivery of computing as a service rather than a product, whereby shared resources, software and information are provided to computers and other devices as a utility (like the electricity grid) over a network (typically the Internet).
From what I know, cloud computing is not a web hosting technique. Cloud computing delivers a service for sharing resources, information and software with other computers. Check out http://en.wikipedia.org/wiki/Cloud_computing for more information.