The Iris
The aperture is usually formed by a number of metal blades (sometimes called an iris.) These are hinged so they can swing to increase or decrease the size of the hole between them.
At their widest, the blades usually do not obstruct the path of light through the lens at all - the lens is said to be at full aperture.

Boke
The number of blades and the shape of the aperture they make differ in different cameras, including diamond shapes, hexagons, heptagons and more, although we normally think of the iris as a circle. The different shapes are sometimes noticeable in the way that small bright highlights give stars of light, and in the shapes of flare we get when bright light sources such as the sun actually shine on the lens. Although it is hard to explain why, odd numbers of blades give a better effect. The shape also affects the rendering of out-of -focus areas; some people talk about the different effects in terms of the 'boke' (a Japanese term, pronounced 'boak-er', and sometimes spelt 'bokeh' as a reminder of this) of the lens. However, the spherical aberration of the lens is also vital in this.