'In photography, bokeh is the blur, or the aesthetic quality of the blur, in out-of-focus areas of an image, or the way the lens renders out-of-focus points of light.'
Found a great tutorial on replicating this effect without a camera in Photoshop. Abduzeedo.com has a great tutorial on this too but I find it easier to watch a video when teaching myself something new. Check this tut out by Ash Davies.
Check out some great examples of real Bokeh here: