In computers, interactivity is the dialog that occurs between a human being (or possibly another live creature) and a computer program. (Programs that run without immediate user involvement are not interactive; they're usually called batch or background programs.) Games are usually thought of as fostering a great amount of interactivity. However, order entry applications and many other business applications are also interactive, but in a more constrained way (offering fewer options for user interaction).
On the World Wide Web, you not only interact with the browser (the Web application program) but also with the pages that the browser brings to you. The implicit invitations called hypertext that link you to other pages provide the most common form of interactivity when using the Web (which can be thought of as a giant, interconnected application progam).
In addition to hypertext, the Web (and many non-Web applications in any computer system) offer other possibilities for interactivity. Any kind of user input, including typing commands or clicking the mouse, is a form of input. Displayed images and text, printouts, motion video sequences, and sounds are output forms of interactivity.
The earliest form of interaction with computers was indirect and consisted of submitting commands on punched cards and letting the computer read them and perform the commands. Later computer systems were designed so that average people (not just programmers) could interact immediately with computers, telling them what programs to run and then interacting with those programs, such as word processors (then called "editors"), drawing programs, and other interactive programs. The first interactive human-computer interfaces tended to be input text sequences called "commands" (as in "DOS commands") and terse one-line responses from the system.
In the late 1970's, the first graphical user interfaces (GUIs) emerged from the Xerox PARC Lab, found their way into the Apple Macintosh personal computer, and then into Microsoft's Windows operating systems and thus into almost all personal computers available today.