My use of guide bushings differs from what you most often read about in magazines and online. Predominantly, articles on router bushings focus around using the router accessory for pins and tails when using dovetail jigs. My use, however, is more outside the box, although I do some dovetail work using the bushings, too.
What are guide bushings? They are small tubes of varying diameters (and lengths) that fit into a router baseplate through which a router bit extends. They are sometimes referred to as template bushings, too.
When run against a template, pattern or dovetail fixture, the side of the bushing keeps the bit spaced a constant distance away (the offset, or the distance from the outside of the bushing to the cutting edge of the router bit). To calculate the offset, take the guide bushing outside diameter, then subtract the bit diameter and divide by two.