All forms of display of information in X are bit-mapped which means that every pixel on the screen is individually controllable.
Therefore we can draw pictures as well as using text (but we do require high quality monitors).
X like most other windowing systems divides the screen into various parts that control input and output. Each part is called a window.
Each window can act as a standard terminal (e.g. we have used Xterm already).
However windows can have other uses such as display of graphics, input from mouse etc.
Not all applications need to consist of a single window.
We can have many windows associated with different parts of one application.
Each subwindow is called a child and are only visible and accessible within the confines of its parent control window.
Note: the background window is the root window - All other windows are children of the root.