The content of the description tag is used more often than not by Google when it displays the results pages for a keyword. It uses only the first 156 characters though so make sure if you are using it to check the length of the text. If you do not check the length your text will be truncated and it will look funny.
To check the length of a piece of text just paste it into a word document and then select the text and then double click the word count box at the bottom of the screen on the status bar.
All meta tags should be at the start of the page inside the head tags.
Google is a little rogue when it comes to using or not using the meta description tag. When it does use it you can have full control of what is exposed to possible visitors but it will often switch what it uses at will with sometimes it just plucking a piece of text from your webpage.
To ensure you have more control over what Google chooses as the text in the search results as well as setting the description tag carefully also make sure that the first 156 characters of text on your page says what you would like to appear in Google.
If you have a flash site or just use pictures with a little text, you have a problem! We have seen Google pick up the alt text from an image and exhibit this when it didn't choose to use our description tag. This will tell you to ALWAYS have some text on your page even if it is only 156 carefully chosen characters (including spaces). - 33376