Intuition is the collective name for the function libraries, data
structures and other elements needed to create a graphical interface for
Amiga applications.  Programmers use Intuition to perform user interface
chores such as opening windows, managing menus, monitoring gadgets,
reading the mouse position and so forth.

Newcomers to the Amiga sometimes think of Intuition as the Amiga's
operating system but it is not.  Intuition is just one component that
together with exec, amigados, and other subsystems make up the whole
operating system.  Intuition is the most visible part of the operating
system though since it provides the graphical user interface familiar to
all Amiga users.

 about user interfaces          how an application sees intuition 
 how the user sees intuition    a simple intuition program