Arrowheads

Arrowheads[s]: specifies that Arrow[] draws one arrow of size s (relative to width of image, defaults to 0.04).
Arrowheads[{$spec_1$, $spec_2$, ..., $spec_n$}]: specifies that Arrow[] draws n arrows as defined by $spec_1$, $spec_2$, ... $spec_n$.
Arrowheads[{{s}}]: specifies that one arrow of size s should be drawn.
Arrowheads[{{s, pos}}]: specifies that one arrow of size s should be drawn at position pos (for the arrow to be on the line, pos has to be between 0, i.e. the start for the line, and 1, i.e. the end of the line).
Arrowheads[{{s, pos, g}}]: specifies that one arrow of size s should be drawn at position pos using Graphics g.

Arrows on both ends can be achieved using negative sizes:

- Graphics[{Circle[],Arrowheads[{-0.04, 0.04}], Arrow[{{0, 0}, {2, 2}}, {1,1}]}]

You may also specify our own arrow shapes:

- Graphics[{Circle[], Arrowheads[{{0.04, 1, Graphics[{Red, Disk[]}]}}], Arrow[{{0, 0}, {Cos[Pi/3],Sin[Pi/3]}}]}]
- Graphics[{Arrowheads[Table[{0.04, i/10, Graphics[Disk[]]},{i,1,10}]], Arrow[{{0, 0}, {6, 5}, {1, -3}, {-2, 2}}]}]

← Arrow

→ Circle