Running software or working on a laptop/desktop is known as desktop computing. Under the desktop computing, you will be having dedicated hardware, operating system and applications. When a single desktop computing machine can't achieve a big task then multiple similar kind of machine kept together to complete the task. This type of computing is known as cluster computing.
Cloud computing is kind of cluster computing where all the machine of different cluster combines and create a cloud of data centers.
A user does not worry about the hardware availability or operation system. Cloud computing gives flexibility to use the resources on need basis and pay only for that.