I will try to explain with what I know which isn't much.

When we use our PC, we access resources we need from our hard disk. In a network, we access resources we need from the various servers.

in cloud computing, we don't have such resources. We access them externally, from a server far away. Sometimes we access them using wireless. So these servers are invisible to us. It is like they are in the clouds.

Just imagine a wireless network with your PC in your country and in our own room. The databas server is in LA while the web server is in NY while the word processing server is in London.

They are not with you. They are in the clouds.

At least this is what I think. Sorry if I am wrong.