HTML meta tags are special tags that are included in the head section of your html that are not directly visible but give additional information on your pages.  These meta tags were very popular at the dawn of the internet and were used quite heavily by the search engines.  That said, everyone wants their site to show up at the top of the search engine results, so many webmasters abused those tags to try to get their pages to the top of the list.  The search engines started to adopt other strategies to avoid this abuse.  The end result is that search engines have their own proprietary methodologies of determining page ranking within search results and these methodologies are closely held secrets.  HTML meta tags are not as important as they used to be for determining your rank in search engine results, but they still play an important part in proper net etiquette.

The approach that I recommend is to always include a basic set of basic meta tags on all your pages.  These may not influence your page ranking in search engine results, but many of these will influence how the search engine looks at your site.  I include the following meta tags in all pages:  title, keywords, description, language, date.created, date.modified, and robots. 

The title of the page and the title meta tag ideally should be the same, and if they are different, they should be fundamentally the same. 

The creation and modified dates help show your visitors and the search engine spiders when the page was created, and as it is indexed over time, how often it is updated.  Although not guaranteed, this may influence how often a search engine will reindex a page. 

Keywords are probably the most abused meta tag.  Many people think that you should throw as many keywords as possible on your page.  This really won't help improve your search engine page rank as most search engines ignore this meta tag, but perhaps it might have some small influence if the rest of your page supports it.  The guidance I give is to use keywords sparingly, and to only use keywords that are truly relevant to the page.  I like to treat them as tags - if you have a special topic in a page, add it as a keyword, and if your entire site is about a particular topic, include that topic in your keywords as well.  But don't spend a lot of time on keywords as that really will not influence your search engine ranking and your time could probably be better used elsewhere.

Description is a meta tag that is supposed to describe the content of your page.  Use it as such, but again, the search engines have their own algorithm to display appropriate content in search engine results, and your description is not likely to be used very often.  That said, it is a standard place to place a common summary of the page, so use it as such.  Keep in mind that public search engines aren't the only thing that looks at your page content.

The language meta tag is useful to describe the language that the page is written in and is a helpful way to help the search engines categorize your content into appropriate languages.

Perhaps the only keyword that is used by search engines, and by that definition, one of the important ones is the robots keyword.  This keyword can be used to tell the search engines that the content on the page is to be indexed (or not), and whether or not you want the search engines to follow other links on the page.  For most sites, it is desirable for most pages to be indexed, but there are a few pages that should not be indexed.  As such, you should include the appropriate robots keyword tag on each page.  This should also be used in conjunction with a valid (and accurate) robots.txt file on your site as well as with appropriate use of rel-nofollow links on other pages in your site.

Keep one thing in mind - search engines are incented to have the most relevant search results for users, and as such, each search company continuously refines their indexing and ranking methodologies to achieve that goal.  As a result, there are no hard and fast rules that you can use to guarantee your placement in the first page of search engine results, except for perhaps providing the most relevant and respected content for those search terms.  You should not attempt to use meta tags to try to elevate your ranking, as you will just be wasting your time.  On the other hand, you should also not ignore meta tags either simply because they don't directly influence your page rankings.  You should consider that meta tags are used to help describe the page and you should have appropriate information in the meta tags that accurately describes your page.

From a CMS standpoint, ADXSTUDIO includes a meta tag server control that automatically brings forward the basic meta tag values from the fields in the standard page editor that is used to manage the page content.  It is typical practice to include this server control in the master page template, which then ensures that all pages in the entire site have a basic set of keywords that are accurate and can be content managed.  The site authors can then use the standard fields in the page editor and trust that every page is output with the appropriate keywords for the page without having to spend a lot of work manually editing the meta tags for each page.  This is a nice balance between good net-etiquette and publishing efficiency.  Of course, you can also add dynamic code to help influence or automate some of the meta tags that are output, but again, be careful to only use meta tags for what they are indended to be - don't try to misuse them because you won't likely improve your search engine rankings and you may be just wasting your time.

I would also like to recommend that you consider that each search engine considers the 'quality' of your page, and some of that is proper page structure (a good reason to be xhtml compliant), appropriate meta tags, internal and external links, indexing mechanisms (like robots.txt and sitemaps) and the content on each page.  Your goal should be to have the best quality page - consider each page part of your reputation.  If you start to get a bad 'reputation' you may find that some of your pages may be excluded from the search engine catalogs (as in not show up at all), and your reputation might even get your site blocked entirely (no pages show up), and may influence how frequently search engines will reindex your site and include new pages.  Your goal should be to have the best reputation with each page on your site using the best practices and following internet standards - that is perhaps your best defense when you are attempting to resolve problems with search engine indexing of your site.