An arrow is specified by its tail and tip. An optional third argument scales the arrowhead.
arrow(P tail, P tip, [double scale]); dart (P p1, P p2); // same as arrow(p1, p2, 0.5); aarrow(P p1, P p2); // double-headed arrow <--->
Pictorially, an arrow consists of a line segment (the shaft) surmounted by a triangle (the arrowhead). In profile, an arrowhead's width is 3pt, and its height is 5.5 times the width. The actual printed height depends on the arrow's orientation with respect to the camera.
By default, an arrowhead is a hollow triangle, which can be colored. The fill() command produces solid, uncolorable arrowheads. PSTricks commands can be used for solid colored arrows.