The \rule command in normal use produces a simple black box:
The depth, width and height parameters are explained in the Boxes chapter.
Here is an example:
This is useful for drawing vertical and horizontal lines.
A special case is a rule with no width but a certain height. In professional typesetting, this is called a strut. It is used to guarantee that an element on a page has a certain minimal height. You could use it in a tabular environment or in boxes to make sure a row has a certain minimum height.
In LaTeX a strut is defined as
LaTeX provides the \hrulefill command, which work like a stretched horizontal space. See the Lengths chapter.