Vignetting is a dirty but useful tool. It allows you to place emphasis where you want after the photo has been taken, but using it is frowned upon. The trick is to get away with it. You tend see three versions, one in which it happens on its own, hipsters just blocking out the corners, and people using more discrete but still noticeable methods in photoshop. Lens vignetting happens when the image circle created by the lens does not quite cover the sensor, causing fall off on the corners. Usually these days you see it with older film cameras especially with the cheaper super wide zooms. It can still happen with the new lenses, especially when filters are stacked. Holgas and other such garbage by purposeful bad design tend vignette. Hipsters love to recreate that effect in photoshop with sloppy black mask and gradient. With more careful adjustments, the same emphasis on your subject can be achieved with more taste and discretion.
The intent on creating a vignette should be to change areas of contrast or to create a new contrast between the subject and the rest of the image. The most basic way to do this is to create a level adjustment and then to mask out the subjected with a simple graduated mask. By creating this mask, you create a contrast between subject and the rest of the image by placing the majority of the brighter tones in the subject. Using a curve layer you can fully remove true whites from the rest of the image again drawing the viewer's eye by placing the stronger contrast in the center of the image. The adjustments do not have to be entirely tone based, but can be based on color as well. Look at the image of the ants, using the movie poster popular blue/orange contrast the image becomes stronger because it feels more dynamic because of the contrast. The ants to begin with were already red orange and yellow dominant, so it was easy to create the contrast by making the area surrounding them green blue and purple dominant. Further since the center of the subject is warm while the outside is cool the ants again become the focus. As teh view's eyes are drawn to the warmer tones first.