This is one of the most common sources for this tag. However, Matt Cutts from Google has indicated that this description can be pulled from other possible sources, including different parts of the page that are more relevant to the keywords or from open directory projects. Generally, though, it is the META tag that becomes the description.
While the title tag should be limited in length, you can use the META Description tag to expand a bit about the content on the page. At a minimum, the description should be a full sentence long. It should not exceed a paragraph. Here are some good META Description tag practices suggested by Google:
Use the description to accurately summarize your page’s content. Since this may appear in the Search Results, it can have a huge impact on the amount of traffic you get from the search engines. Avoid stuffing this with keywords that aren’t relevant to the page. Also, be sure to make it readable as it could be the first piece of content that is visible to your potential visitors.
Create a different, unique description for each page on your site. Since each page is different, it only makes sense that the descriptions would also be different.
The Destination URLThe next piece of the snippet is the URL that you will land on. Notice how the page name is bolded because it matches the search term. Generally, it is a very good practice to name your pages with your main keywords to help Google know what the page is about.
The Cached and Similar LinksThe last couple of pieces in the snippet are the
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