A light guide is a little more complicated and a lot harder to design than a “light pipe” (see below). Both these structures bring light from an LED in one place to an observer’s view in another place. The difference is, with a light guide, you’re usually trying to spread the light out in a larger pattern than you would with a light pipe going to a tiny indicator button. Fancy DRL’s (or daytime running lights) on cars and trucks would be one place you might find these. Commercial displays can be another, or anywhere you want an even glow of light across a larger-than-button sized surface.
Like light pipes, light guides can (and should) be designed with optical simulation software. Light guides are a lot harder to design well! Often the optical engineer introduces a bunch of carefully calculated slivers of light leakage along the light guide. These small features need to be balanced with the larger curves of the overall shape to keep the appearance evenly lit. Like a light pipe, light guides are also usually molded from a clear plastic.