The description meta tag is one of the only meta tags, which can have a small influence on your performance in the SERPs. If not directly, then at least indirectly.
People are much more likely to click results, if they see a carefully chosen description, rather then a automatically generated one, which is more likely to be less on target.
The description in the SERPs
The description shows up just below the title of your page, and is often just a generated chunk from the page. If you don't have a meta description, search engines will generally just show a chunk of the page, containing some of the words used as the search string.
Below is a screenshot of how Brugbarts SEO Tutorial looked in Google at some point.
Choosing a good description
Its very important that you chose a good description. A page with a bad description, which is ranked #1 on Google, may get less clicks then a page ranked further down in the results with a good description.
Its fine to include promotional phrases in the Title, but keep in mind that it may decrease the relevancy for your page, when run against certain Keywords and Keyphrases. If you are including promotional text in your title, be sure to include it last in the title, like done below.
SEO Tutorial - Brugbart.com
A good title is a clean title, an even better one then the above is:
How to insert a description meta tag
The description meta tag should be placed in the head section of your page.
<meta name="title" content="The Description Meta Tag">