You want to go into your pages meta information and change it there. That will help. Make sure your headers say the keywords you are trying to optimize your page for. From the Horse's mouth. http://www.google.com/support/webmasters/bin/answer.py?hl=en&answer=35264
Google call it a snippet. Usually they take the information from your meta description tag, or if you have a dmoz entry, from that. They may use some text from your page for specific keyword searches, there's nothing you can do about that.
You can access the description meta tags of your web pages either through using page building softwares which would be very easy and convenient or simply using the Notepad of your Windows and opening the Html pages and then modifying the description lines.
Site description in Google comes from your meta description tag, if you have it (if you don't have it, Google will first check if your site is in ODP and grab that description or if you are not in ODP then it just searches for words in your site where the keywords are) For example, here is what Target.com has in Google Welcome to Target Expect More, Pay Less at Target.com, the official Target online retail site. Shop the latest in Women, Men, Baby, Kids, Home, Bed+Bath, Furniture, Sports, ... www.target.com/ - 168k - Cached - Similar pages - Note this And here's their metatag -- note that the description tag is exactly the same as the Google description <title>Welcome to Target</title> <meta name="description" content="Expect More, Pay Less at Target.com, the official Target online retail site. Shop the latest in Women, Men, Baby, Kids, Home, Bed+Bath, Furniture, Sports, Toys, Electronics, Health and See All. Check out current deals and promotions, and the Weekly Ad, too." /> So if you want to change the site's description, either: 1 - put a meta description tag if your site has none 2 - change your meta description tag