From 'Operating System Concepts, 3rd Edition' by A. Silbershatz, J. Peterson, and P. Galvin: In the section introducing "multiprogrammming," the book makes an analogy to try to make things clearer:
[..discussion of CPU switching from job to job instead of waiting for I/O to finish on current job...]
"This idea is quite common in other life situations. A lawyer does not have only one client at a time. Rather, several clients may be in the process of being served at the same time. While one case is waiting to go to trial or to have papers typed, the lawyer can work on another case. With enough clients, a lawyer never need be idle. (Idle lawyers tend to become politicians, so there is a certain social value in keeping lawyers busy.)"