When representing a 3D-figure on a flat surface, you are making a projection of the figure on the surface. This often gives distortion. When making a "parallel" projection:
edges parallel to the drawing surface are depicted full-scale;
parallel edges are depicted parallel;
edges that are parallel and of the same length are depicted with the same length (a.o. this means that the center of a line segment also is in the center of the figure).
Here you see a "parallel" projection of a cube $ABCD.EFGH$. The edges $AB$, $BF$, $FE$, $AE$ and $DC$, $CG$, $GH$, $DH$ are parallel to the drawing surface and therefore full-scale. The squares $ABFE$ and $DCHG$ are square in the drawing as well. The edges $BC$, $FG$, $AD$, $EH$ are parallel in the drawing as well. They have been shortened to make the figure really look like a cube. On a blank page you work with an angle and a scaling factor.
You will see the most important solids by clicking in the figure.