It doesn't try to account for every single page on the site. Instead, it's a solid summarization of the site's primary content. As you can imagine, the description for your home page will be broader than the ones you might use on subpages. WebPromote suggests that you keep your description at less than 25 words. Thinking that you'll really get noticed if your description is the longest one returned, you might be tempted to put in a whole paragraph or more, but don't waste the kilobytes.
Search engines use a set amount of characters in the descriptions they return.
If yours is too long, it will be cut off abruptly. The rule of thumb is to try and create a sentence that will make users want to click through to your site. Crafting a simple sentence that can do that effectively will serve you better than rambling your way through several lines in an attempt to list everything, including the kitchen sink, about your site. You can always revise and add to your keywords listing. By filling in the relevant terms, phrases, and concepts related to your site, you increase your chances that people searching for the information you have will find you.