Normally, you use one computer and one or more programs to do your work.
In a cloud, you have an entire network of machines to work with. You will access the cloud from one machine, machines in the cloud will find a machine that is the least busy, and allocate your work to it. At any time, the machine you work with in the cloud can change; you would not notice the difference. Additionally, for more complex tasks, many machines in the cloud could be doing the work in pieces, again, you would not notice.
Cloud computing is basically a set of machines that function as one unit, with different machines for the purpose of dispatching work and others for actually doing it.