Bokeh is the blurring effect created by a camera lens on the out of focus areas of the frame. It comes from the Japanese word “boke” meaning blur.
Bokeh variations depend on the lenses’ mechanical design and depth of field (DOF).
Photographers can also get very creative and use various shapes in front of the lens to create original bokeh such as using this very affordable DIYP Bokeh Masters Kit. Here’s a sample:
Check out Wikipedia for the complete technical description.