Sometimes, When I tell people I'm a Graphic Designer, folks get confused. It makes sense, What do I know about any other profession? Not much. Folks usually end up saying something awesome to me like, "You must be so good at the Photoshop!" Sometimes I want to go on forever about how that's not really what it's all about, but I usually end up saying ... "Yes, I AM good at the Photoshop", just to avoid a long conversation about something that maybe they aren't really interested in anyway.
... But, I am gonna try and clear up one of the most elusive graphic design terms out there, the "Style Guide". Style Guides are one of the things I end up working on the most and they are one of the VERY most confusing things to explain to a non-graphic designer, so here is an attempt:
A Style Guide is essentially a bundle of art and branding elements organized into a guide, and then handed off to a client, so that they can have consistency within their brand.
Here is a more specific example: Mattel wants to refresh their Polly Pocket Brand. They hire a graphic designer. The graphic designer starts by researching graphic, color, product, and apparel trends for girls in the 4-9 year age range. The Graphic Designer also will familiarize themselves with the Polly Pocket Brand, and the clients specific needs. Based on all of that information, art is developed. This includes typography selection, color palettes, patterning, character art, graphic compositions, and packaging layouts. A guide is then developed to show how these art elements should be used, and how they can be transformed into actual product. After Mattel approves all of this art, they then hand the guide off to a place like Target or Toys-R-Us, who then follows the guide and produces a consistant and fresh new line of products and packaging for Polly Pocket.
There it is in a nut shell.