See section 12.5.3 for segments in space.
The segment command and the Line command draw line segments. (The segment command can also draw vectors, see section 11.7.4.)
segment takes as argument two points or a list of two points.
segment returns the corresponding line segment and draws it.
Line takes as argument four real numbers, the first two represent the coordinates of the beginning of the segment and the last two represent the end.
Line returns the line segment and draws it.