A construct placed in the HTML header of a web page, providing information which is not visible to browsers. The most common meta tags (and those most relevant to search engines) are KEYWORDS and DESCRIPTION. The KEYWORDS tag allows the author to emphasize the importance of certain words and phrases used within the page. Some search engines will respond to this information - others will ignore it. The DESCRIPTION tag allows the author to control the text of the summary displayed when the page appears in the results of a search. Some search engines will ignore this information.
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Meta tags have been considered the "saving grace" of many websites over the past years. Having Keywords and Description tags was often given more consideration than much of the remaining page. Then the major search engines like Google and Yahoo! got wise!
Before the world of SEOs and web designers decide to hang me up by my toes, let me set a few things straight!
Google still uses the Description tag from time to time. Also, there are still many secondary search engines and directories that use and/or require both tags mentioned above. All in all, you still want to have these tags within each web page but, they no longer hold the prominence they once did.
Now you have to work for it!
In today's competitive online world, the Title tag is the most important on-page content there is! Bar none! No exception! Amen!
After the Title tag comes the Heading tags, emphasis tags (e.g. bold, strong, italics) and then the page content itself including the density of your keywords/key phrases throughout the page. It is best to have these keywords/key phrases as close to the top of the page content as practicle to make it easier for search engines to find and index.
Then comes the names of your pages themselves and the folder naming conventions of the site. Not the least of this side of optimization is the actual domain name itself!
As you can see, there are a number of things to consider which is why it's important to hire a Professional SEO when you want it done right.
Like when your business livelyhood is on the line!
This only touches the surface regarding on-page optimization. Then there is link popularity but...that is a whole other story.