When you photograph highlights out of focus they create blobs of colour. The shape and size of the blob is dependent on the aperture configuration of the lens and the focusing point.
They best looking ones are with lenses with more blades so a circle is created.
I bought a couple of red Christmas candles for a specific shot, and here it is.
I defocused the Christmas lights and set the camera up at the defocused distance. I then placed the candle in the line of fire so it was sharp.
I then moved the camera position (left right up down) so the candle was in the best place in the frame. This is trying out some old Canon FD lenses on the Olympus E-P2.
The main one for my photo365 has round Bokeh created with a very old breach lock 100mm f/2.8. I've also included one taken with the 500mm mirror lens (giving a 1000mm effective focal length.
Mirror lenses are known for their doughnut shaped highlights and this shot shows that off to the full.