The descriptions search engines offer when displaying search results are generally retrieved from two sources:
a) displayed page's title tag;
b) displayed page's description meta tag or, in default of same, first characters of page's body text; number of characters displayed is limited, with some engines picking up a maximum of 150 characters, other offering slightly more;
c) displayed page's keywords meta tag - while this meta tag's content will not be excerpted for displayed text's description proper, it is one of several factors determining which search results are displayed at all and in which order (ranking). (Note that this is a generalization - some, though few, search engines refuse to take into account any meta tags. Obviously, different rules apply in their case.)
Both a), b) and c) should relate to specific page's content, not web site's or its corporate owner's overall theme! That is why they are placed individually in each page's header in first place.
There is a popular misconception amongst web marketeers regarding search engine positioning mechanics, namely that web page meta tags and titles are to be employed as instruments of branding. However, if true at all, this would typically apply exclusively to a web site's main or index page. One of metaphors commonly used in propagating this erroneous marketing policy is that of "business card".
While it is true that a search result functions as a site's public representation it must also be noted that this should always relate to specific page displayed: anything else may legitimately be deemed diversionary tactics, meaning that page could be penalized for "spamming". (Yes, respective search engines' definition of "spam" varies widely and is all but consistent. Also, in several cases it notably lacks a consistent logical basis, but that is not topic at discussion here.)
This aside, it should be remembered that it will be both, a page's title and its description which will induce a searcher to actually click on link and visit site.
Hence, a page's description as displayed in search results is more akin to a product precis or summary than a general corporate business card and should be construed that way.
A unified approach, presenting one and same promotional text on each and every page displayed by search engines, while seemingly making sense from corporate image point of view, actually constitutes a severe and unnecessary self-restriction, effectively hampering overall online marketing efforts.
A practical example -------------------
Let's assume that you are running a used car dealership with an online presence (web site). Your company's name is "Honest John Autos Inc." and your main corporate marketing slogan which made you famous all over your home town is "Used Cars Galore: The Fairest - The Squarest - The Best!(TM)"
You offer a fairly extensive variety of used cars in your products palette, ranging from farmers pickup trucks to vintage American autos, foreign luxury and sports cars, etc.
Your web site has some 150 pages, all of which you will submit to search engines for indexing. The pages are well focused and carry specific titles, e.g.:
- "50s Chevvy Beauties" - "As good as new - Oldsmobile special offers" - "Luxury finally made affordable - Porsche Paradise" - "Agro Cars - Pickup Center" etc.
Now if you insist on putting your "Used Cars Galore: The Fairest - The Squarest - The Best!(TM)" slogan in every page's description tag, all you will be able to rely on to pull visitors to your site is your page title. But while it may appear to you that slogan is a nice marketing reinforcement of page title "Agro Cars - Pickup Center", fact is that you might as well qualify title message with a specific description which is a lot more to point in relation to title - and to surfer's original request.
Hence, you might wish to describe your Agro Cars page in a more focused manner, e.g.:
----------------------------------------------------- "California's largest selection of second hand agricultural pickup trucks - excellent condition, and no-questions asked 30 days full refund guarantee!" ----------------------------------------------------- (150 chars.), or similar.