Tags defining website title, description and keywords used for locating engine spiders or crawlers for, fast indexing in search engines, to appear on organic listing by, searching for any keywords or key phrase for a website are called meta tags.
There are different types of meta tags like Title tag, Meta Description tag, Meta Keywords tag, Meta Robots tag, Meta Revisit tag, Meta Expires tag, and Meta Refresh tag.
Meta tags do not guarantee a top ranking on crawler-based search engines. Today, the most important use of meta tags is that they help in controlling to some degree web pages description by some search engines, as well as help prevent pages from being indexed at all.
The Title Tag: The text placed in the HTML title tag title appears in the reverse bar of someone's browser when the web page is viewed. The title tag is also used as the words to describe your page when someone adds it to their "Favorites" or "Bookmarks" lists. Title tag for Search Engines is very crucial. The title tag helps the search engine to decide the ranking of web page.
The Meta Description Tag: The meta description tag allows you to influence the description of your page in the crawlers that support the tag. Meta Description tags are useful for your pages, as it gives you some degree of control with various crawlers. Often first sentence or two of body copy from web page are taken for meta description tag.
The Meta Keywords Tag: Crawler-based search engines are provided additional text to index along with the body copy. Meta keywords tag doesn't help much as they are ignored by most of the crawlers. However sometimes it is useful by serving as a way to reinforce the terms you think a page is important for. It is also important as it helps your page come up for synonyms or unusual words not appearing on the page itself.
Robots meta tag: When you want a page not to be indexed by a search engine this tag is used. To do so, simply add this text between head tags on each page you don't want indexed. Most major search engines support the meta robots tag. The meta robots tag also has some extensions offered by particular search engines to prevent indexing of multimedia content.
One should remember the following about the different Meta tags:
1. Meta Robots: This tag enjoying full support from search engines is used only when you do not want search engine to index a particular page.
2. Meta Description: This tag is worth using as it enjoys much support from the search engines.
3. Meta Keywords: This tag is not worth the time to implement as it is supported only by some major crawlers.