For example, when a driver (car operator) uses the horn to warn a pedestrian, then the driver is the horn operator and the pedestrian is the audience. 
Other examples of various types of users are the operators of electronic boards, such as in airports or in eCommerce, who advertise the messages, and the audience, who read the messages...
This term is distinguished from users in computing (see User (computing)), in which the user is often a piece of software, called agent that needs another piece of software for its operation. This kind of terminology is typical of computer operation, which adopts terms used in daily human activities (files, programs, program entry and exit, quitting, abortion, desktop, mouse, and more).
Unlike consumer products, the customers of systems are not the same as the users. Typically, the customer is often an organization, and the user is an employee of that organization. This implies, for example, that in case of significant loss due to use error, the customer seeks to nominate the user as accountable for the loss.