The Life of a Process
The life of a process is somewhat analogous to a customer at the post office. The clerk is like the CPU and the scheduler combined. When customers enter the building, they are like new processes. As they enter the line they are like jobs in the ready queue. Once they are called up to the counter, they are 'being processed', or in a running state. If the customer completed all required forms correctly, the clerk may complete every transaction (sending a certified letter, buying some special edition stamps, sending Priority Mail, etc.,) and the customer leaves (the process is terminated). However, suppose they also need a visa. The postal clerk (CPU scheduler) has to interrupt the process (after only completing the certified mail) and send the customer to the visa desk (which is like an I/O device). There's a line at that desk and the customer has to get at the end of the line, like a process being sent to the waiting queue. In the meantime, the postal clerk continues to work by calling the next 'process' up to the window. Finally, after some time, the first customer gets his visa, and returns to the ready queue. However, after the clerk completes the current customer's transaction (terminating that process), he calls the one with the visa back to his window, in front of the next person in line. He completes the Priority Mail and limited edition stamps transactions, and that process is finally done.