These circles are defocused images of small bright points. You can make a test with a led diode or another point light source. Focus far behind it and with the same lens and aperture you should see similar circle, but just one.

The actual look of these circles depends on the lens design. They are pretty much rounded here, which suggests that the lens was either wide open (no aperture blades in the way) or that the aperture in the lens has rounded blades. The ratio of brightness in center vs. edges of these blur circles depends on level of aberration corrections in the lens. Under or over corrected lenses tend to have uneven brightness in the circles, for example patches behind the focusing plane on pictures taken with lens that suffers from spherical aberration has these patches bright in the center, but dissolving towards the edges. The same lens would have circles that are brighter at the edges with blur in front of the focusing plane. Evenly lit circles would be characteristic for well corrected lenses, even though, paradoxically, this can create some not so great bokeh under certain circumstances... The fact that the shape of the blur patches does not appear circular is likely due to optical vignetting - some of the light is physically obscured by parts of the lens or the camera chamber. The effect is sometimes called cat's eye.