META: "the necessity to designate a new but related discipline designed to deal critically with the original one." META tags are rudimentary indicators designed for search engine spiders to "deal critically,"or use as basic information for indexing your site in their database.
How are Meta tags useful
META tags are a useful way to summarize the contents of a web page and to facilitate the correct cataloging of that page within major search engines without the data really being seen by the user.
How everything works
When the spider of a search engine locates your web page it will need to index it, adding it to its searchable database. Some search engines will index all of the words in your document and will use the first 250 words as a short abstract to index your site. In the absence of any of this information, META tags help search engines index your site by providing specific information as the META tag source.
When someone searches
When someone performs a search request at a search engine that supports META tags and they query for a keyword or a phrase related to your site, your page may show up in the list of results by its title. Underneath the title will be the first one hundred or so characters of the description you placed in the META tag.
Bringing it to the front
It is possible for you to control how your page is indexed by using the META tag to specify additional keywords to index. META information can be used to improve the listing of documents by providing information that is not normally displayed when a document is loaded into a browser and cause more pertinent traffic to be directed to your page. This information helps your visibility when it is extracted by search tools for use in identifying, indexing, and cataloging documents.
Keep it simple
The best tags are not ones repeating keywords the most, the ones with every single possibile words related to your site that you can think of, or the ones with words unrelated to your site. The best tags simply reinforce what is already on your pages.