To maintain data and applications that uses the internet and central remote servers technology is called as Cloud computing. Cloud computing allows clients and businesses to utilize applications without installation and access their private files at any computer with internet access. example of cloud computing is Gmail or Yahoo email etc.
simple possible way is Google docs is a kind of cloud computing. nothing to install in hard drive just use it online.
It just astounds me the number of people who still think they're answering the OP's question after six months.
files can't be downloaded to your hard drive if you will bee using those cloud computing OS, it this true? its what these guys are talking about in this thread http://www.cebutechforum.com/Thread-cloud-computing