What are Meta Tags - MetaTags in HTML

These are HTML or XHTML tags that provides information about the web page. Meta tags may contain information about what the page is about (keywords and description), language, author, when it was write and much more. Many search engines use this information when indexing pages, because these tags help the search engines to understand the contents of a site. Putting correct Meta tags in the header part of an HTML document is called on-page optimization in Search Engine Optimization world.

Meta tags can have two possible attributes:

<META HTTP-EQUIV="name" CONTENT="content" />
<META NAME="name" CONTENT="content" />

Following are some commenly use meta tags with brief explantion and examples of how they can be written well:

Page Title : The page title is not a real meta tag but it's worth considering it here in relation to them. It is one of the most important element and every document must have it.
e.g. <title>What are Meta Tags - Tips and Tricks</title>

Meta Description Tag : The meta description tag is intented to be a brief and concise summary of your page's content. Most of the search engines index the meta description tag found in the head section and present these contents as result of a search query.
e.g. <meta name="description" content="The meta tags are normally found in the head section of an HTML document" />

Meta Keywords Tag : A meta keywords tag can be considered as a brief and concise list of the most important themes of your page. This tag has little to no relevance with the major search engines today. 
e.g. <meta name="keywords" content="Meta Tags, Meta-Tags, Tips, Meta Elements" />

Meta Robots Tag : The meta robots tag lets you specify that a particular page should NOT be indexed by a search engine. This tag can be used if the users cannot provide the robots.txt file on their website. Some common usage expmples are given below. 
e.g. <meta name="robots" content="noindex" />
e.g. <meta name="robots" content="nofollow" />
e.g. <meta name="robots" content="noindex, nofollow" />
e.g. <meta name="robots" content="noindex, follow" />
e.g. <meta name="robots" content="index, nofollow" />
e.g. <meta name="robots" content="index, follow" /> (This is default)

Below are examples of robots meta tags for specific search engines.
For Googlebot : <meta name="googlebot" content="noindex, nofollow, noarchive" />
For MSNbot : <meta name="msnbot" content="noindex, nofollow" />

Meta Language Tag : The meta language tag is usually used to specify the natural language of the html document. 
e.g. <meta http-equiv="content-language" content="en" />

Meta Content Type Tag : This is another important meta tag that can be used to provide information about the character encoding of the contents of a particular web page. It is recommended to always used this tag (even with the previously-default charset ISO-8859-1). 
e.g. <meta http-equiv="content-type" content="text/html; charset=utf-8" />

DC Dublin Core META Tags (DCMI) : The Dublin Core metadata element set is a standard for cross domain information resource description. 
e.g. <meta name="DC.title" lang="en" content="DC Dublin Core META Tags - DCMI Dublin Core Metadata Initiative" />

Using the Dublin Core syntax for your meta elements instead of developing a custom vocabulary provides a consistent and standard approach to describing the content within your site. The Dublin Core syntax may be combined with standard meta tags with no problems. Dublin Core is supported by various search engines (such as Ultraseek, for example) that may be used to add search features to your own site.