So just what is a brand identity style guide, or standards manual, as it's sometimes called? Such a document defines a corporation's or institution's (or even a country's) graphical identity, including its logo and how it's used, as well as logo straplines, fonts, colors, numeric styles, you name it. Such guides can be extremely thorough and extend to concrete examples of how these elements must be used for such things as business cards and letterhead, as well as for digital media.
The idea is to ensure a graphical consistency that will unfailingly support the brand identity. The brand guidelines for the Cunard cruise line express this very clearly: "This information has been designed and presented to ensure that the Cunard brand is marketed in an appropriate and consistent manner. It is essential that the heritage, quality, elegance and reliability of one of the world’s leading brand names is upheld and adhered to in all circumstances." Cunard employees, whether designers or those simply employing design elements for creating marketing collateral or other documents, thus know exactly what to do and not do.
Edward Tufte, who knows a thing or two about the importance of clear and consistent design, throws some additional light on this: "In a large corporation, it is very hard to get hold of graphic design. Thousands of employees are putting vast amounts of visual material out for public consumption — forms, advertising, interfaces, manuals, and so on and on. That material represents and exemplifies the corporation and is the design presence, indeed often the very expression, of the corporation along with the products themselves. If there is a well thought-out design standard, it should be followed. In practice, great design comes from great designers. That is empirically the case. If a great designer did a first-rate standard, that model should be followed. Great design is not democratic; it comes from great designers. If the standard is lousy, then develop another standard." It's worth reading his entire statement on his site.
If you're currently in the market for a new logo, you'd do well to spend some time looking at existing guidelines, to better think through how your new logo will be consistently used. And if you're a designer? Well, guidelines are no less than a gold mine of design and deployment practices. Close study of these amounts to the equivalent of an entire graphic design education. So where do you find them? While some are only available in printed form or via corporate intranets, a surprising number can be found simply by searching. Or you can start by working your way through lists that others have compiled. A great place to begin is with one of the most recent of these, a collection of sixty guidelines created by brand identity designer David Airey. I've included a few examples from style guides below to give you an idea what to expect.