Closely related to Depth of Field, bokeh is a term for the quality of the out of focus parts of an image - not how blurry it is, but what that blur is like.
Preferences vary, but when most people refer to good bokeh, they mean it’s smooth, creamy, with very little pattern in it. Personally, I quite like a bit more texture in the bokeh.
When people just describe a lens’ bokeh as good, that’s a matter of taste, so it may not be what you think is good. Bokeh with a lot of texture, or even shapes and patterns in it can be quite ‘arty’, but can also be distracting from the subject. For portraits, for example, you really should want the attention on the person you’re photographing. Should. I do love some messy and weird bokeh myself.
See [[Lomography Lenses]] for some really weird bokeh in new lenses. For a much cheaper alternative, Helios lenses are old Soviet lenses that can give a swirly look.
Why would you want swirly bokeh? Would you want it? I love it, but I’ve seen people say it makes them feel sick to look at. If you fall in the latter camp, you probably don’t want it.
Bokeh generally turns points of light into circles. Sometimes the ‘circles’ are more hexagonal, or pentagraminal (or whatever the word should be). I’ve even heard of lenses making triangles. It’s usually down to the shape of the aperture blades, unless they’re wide open. Anyway, some lenses squish those circles, making them more elliptical. And they are squished perpendicular to a line from the centre of the frame. Do that enough to a whole bunch of circles, and you end up with a pattern that looks like a circle around the centre point.
As long as the background is patterened, light and dark, or different colours the result looks like a swirly pattern around the centre, but only in the out of focus parts.
Stick someone’s face in the middle of the frame, with the right background, and the result can be pretty striking. Here:
That’s Mirela, with a background of autumn leaves. Taken with my Lomography Petzval, with the swirl turned up to max. Yes, that lens has a dial for controlling the level of swirlyness - the Bokeh Control Ring. The people at Lomography are completely nuts, in a good way.