Meta tags help the search engine spiders collect information for search results. The spider "robots" go out and index your pages periodically, the times they search are set up by the search engine companies. Some spiders pick up the first 150 "characters" (letters) of a page and use it for a description. If you have meta tags in place, the spider usually will use these as your page description in the search engine results (a good example are HotBot results). As is often the case, the search engines and directories have different rules.

Meta tags are helpful, but they are no guarentee of a high placement in search engine results. It's a good idea to include them on every page of your web site. You can individualize the tags to describe each page to make the most of your meta tags.

The title tag is important, I always put the company name in the title; you can choose to include your company url as well. Use a few keywords (what you might use to search on to find your site) in the title, be sure they are also contained in the page as well. Make each page's title unique to the page's content.

Descriptions should be kept to 150 to 200 "characters". Keep your description short, describe the site and the services you offer without hype. Find the best way through your description meta tag to encourage people to visit your site.

Keep your keywords in lowercase, some search engines are case sensitive. Your keywords can be up to 1,000 "characters". My preference is to come up with the best combination of keywords with a combination of single words, phrases (which help decrease the number of search results) and include the company name. Have your most important keywords in the first ten words of the keyword meta tag, I usually include the company name in the first ten words. Keywords should match what is in the content of your page. There are varying opinions on use of spaces and commas in your keywords, as well as repetition of keywords (some say no more than three, some say seven.) Research and experiment in finding what works best for you.

Your best web sites for research information on meta tags are Danny Sullivan's Search Engine Watch article on Meta tags and visiting the Search Engine Forums Meta Tags Forum. Both are excellent resources for more on Meta tags.

Here's a handy tool to use, very helpful in finding popular search terms:
GoTo search Terms Suggestions