By saying "Cloud" most people mean any kind of data hosted in a data center or any services delivered through Internet, from any data center. It is funny to all of us who have been in this industry for ages, as the IT Hosting is always been like that.
Security on the Cloud is very broad discussion. It starts with power stability, continues with network security and then goes into specific niches. The end users need to deal with the security in their own servers (instances).