Polylines are very important in AutoCAD because they are treated as single entities for many operations. For example, a single command can be used to fillet each corner of a polyline or to form a smooth curve from a polyline. Further, the area and perimeter of a polyline can be also calculated. Finally, polylines can be exploded into separate line and arc entities when necessary. This command is used to draw a regular polygon.
Polygons are in fact closed polylines and can be treated as such, once constructed.
This command is used to draw a cross-hatch pattern within a closed boundary or between closed boundaries. If the boundaries are not well defined, then the hatching can leak out in quite unexpected ways. AutoCAD comes with an extensive set of pre-defined patterns that can be selected. Thus, when you are prompted for a scale factor, you need to visualise the size of a 1x1 square relative to the area you wish to fill in order to estimate the required scale to create the effect you want. This command is driven by a dialogue box and provides a great number of options, including the ability to preview the hatching before committing yourself.