The default aperture is circular; we can see this by using a small, bright point that is out of focus. Ordinarily, a point this out of focus would not be visible - its light would be too spread out over the film plane - so we have to make it artificially bright to show up (RIB for example). Yielding this image:
[0 0 0 0]
By setting the sides to 5, the aperture becomes a pentagon.
[5 0 0 0]
Setting the angle to 36 rotates the aperture counterclockwise by 36 degrees.
[5 36 0 0]
A positive roundness value makes the sides of the pentagon bow outward.
[5 36 0.5 0]
A negative roundness makes the sides of the pentagon bow inward. Note that because the light is spread over a smaller area, the bokeh here is somewhat brighter.
[5 36 -0.8 0]
A positive density makes the aperture transmit more light at its edges.
[5 36 -0.8 0.7]
A negative density makes the aperture transmit more light at its center.
[5 36 -0.8 -0.7]
We can also modify the eccentricity of the "ellipse of confusion"--the region on the film plane hit by samples from an object that is not in focus--using the dofaspect parameter. A value greater than 1 makes the blur bigger horizontally (e.g., 2.0 here):
A value between 0 and 1 makes the blur bigger vertically (e.g., 0.5 here). This emulates the oval-shaped defocus blur produced by an anamophic anamorphic lens.
Though not directly controllable as a setting, it's worth noting that when a source of light gets close in size to the circle of confusion, its shape will also begin to have a significant effect on the shape of the bokeh. For example, if we had seen a larger square instead of a tiny point through the default circular aperture, the bokeh would be shaped like this (shown without tonemapping in order to emphasize the shape):