In one form, meta elements can specify HTTP headers which should be sent before the actual content when the HTML page is served from Web server to client. For example:

This specifies that the page should be served with an HTTP header called 'Content-Type' that has a value 'text/html'. This is a typical use of the meta element, which specifies the document type so a client (browser or otherwise) knows what content type to render.

In the general form, a meta element specifies name and associated content attributes describing aspects of the HTML page. For example:

In this example, the meta element identifies itself as containing the 'keywords' relevant to the document, uCoz and Web Hosting

Meta tags can be used to indicate the location a business serves:

In this example, geographical information is given according to zip codes.

Meta element used in search engine optimization

Meta elements provide information about a given Web page, most often to help search engines categorize them correctly. They are inserted into the HTML document, but are often not directly visible to a user visiting the site.

They have been the focus of a field of marketing research known as search engine optimization (SEO), where different methods are explored to provide a user's site with a higher ranking on search engines. In the mid to late 1990s, search engines were reliant on meta data to correctly classify a Web page and webmasters quickly learned the commercial significance of having the right meta element, as it frequently led to a high ranking in the search engines — and thus, high traffic to the website.

As search engine traffic achieved greater significance in online marketing plans, consultants were brought in who were well versed in how search engines perceive a website. These consultants used a variety of techniques (legitimate and otherwise) to improve ranking for their clients.

Meta elements have significantly less effect on search engine results pages today than they did in the 1990s and their utility has decreased dramatically as search engine robots have become more sophisticated. This is due in part to the nearly infinite re-occurrence (keyword stuffing) of meta elements and/or to attempts by unscrupulous website placement consultants to manipulate (spamdexing) or otherwise circumvent search engine ranking algorithms.

While search engine optimization can improve search engine ranking, consumers of such services should be careful to employ only reputable providers. Given the extraordinary competition and technological craftsmanship required for top search engine placement, the implication of the term "search engine optimization" has deteriorated over the last decade. Where it once implied bringing a website to the top of a search engine's results page, for some consumers it now implies a relationship with keyword spamming or optimizing a site's internal search engine for improved performance.

Major search engine robots are more likely to quantify such extant factors as the volume of incoming links from related websites, quantity and quality of content, technical precision of source code, spelling, functional v. broken hyperlinks, volume and consistency of searches and/or viewer traffic, time within website, page views, revisits, click-throughs, technical user-features, uniqueness, redundancy, relevance, advertising revenue yield, freshness, geography, language and other intrinsic characteristics.
The keywords attribute

The keywords attribute was popularized by search engines such as Infoseek and AltaVista in 1995, and its popularity quickly grew until it became one of the most commonly used meta elements.[1] By late 1997, however, search engine providers realized that information stored in meta elements, especially the keywords attribute, was often unreliable and misleading, and at worst, used to draw users into spam sites. (Unscrupulous webmasters could easily place false keywords into their meta elements in order to draw people to their site.)

Search engines began dropping support for metadata provided by the meta element in 1998, and by the early 2000s, most search engines had veered completely away from reliance on meta elements. In July 2002, AltaVista, one of the last major search engines to still offer support, finally stopped considering them.[2]

No consensus exists whether or not the keywords attribute has any effect on ranking at any of the major search engines today. It is speculated that it does, if the keywords used in the meta can also be found in the page copy itself. With respect to Google, thirty-seven leaders in search engine optimization concluded in April 2007 that the relevance of having your keywords in the meta-attribute keywords is little to none[citation needed] and in September 2009 Matt Cutts of Google announced that they are no longer taking keywords into account whatsoever.[3] However, both these articles suggest that Yahoo! still makes use of the keywords meta tag in some of its rankings. Yahoo! itself claims support for the keywords meta tag in conjunction with other factors for improving search rankings.[4]
The description attribute

Unlike the keywords attribute, the description attribute is supported by most major search engines, like Yahoo and Bing, while Google will fall back on this tag when information about the page itself is requested (e.g. using the related: query). The description attribute provides a concise explanation of a Web page's content. This allows the Web page authors to give a more meaningful description for listings than might be displayed if the search engine was unable to automatically create its own description based on the page content. The description is often, but not always, displayed on search engine results pages, so it can affect click-through rates. Industry commentators have suggested that major search engines also consider keywords located in the description attribute when ranking pages.[5] W3C doesn't specify the size of this description meta tag, but almost all search engines recommend it to be shorter than 155 characters of plain text.[citation needed]
The language attribute

The language attribute tells search engines what natural language the website is written in (e.g. English, Urdu or French), as opposed to the coding language (e.g. HTML). It is normally an IETF language tag for the language name. It is of most use when a website is written in multiple languages and can be included on each page to tell search engines in which language a particular page is written.[6]
The robots attribute

The robots attribute, supported by several major search engines,[7][not in citation given] controls whether search engine spiders are allowed to index a page, or not, and whether they should follow links from a page, or not. The attribute can contain one or more comma-separate values. The noindex value prevents a page from being indexed, and nofollow prevents links from being crawled. Other values recognized by one or more search engines can influence how the engine indexes pages, and how those pages appear on the search results. These include noarchive, which instructs a search engine not to store an archived copy of the page, and nosnippet, which asks that the search engine not include a snippet from the page along with the page's listing in search results.[8]

Meta tags are not the best option to prevent search engines from indexing content of a website. A more reliable and efficient method is the use of the Robots.txt file (robots exclusion standard).
Additional attributes for search engines
NOODP

The search engines Google, Yahoo! and MSN use in some cases the title and abstract of the Open Directory Project (ODP) listing of a Web site for the title and/or description (also called snippet or abstract) in the search engine results pages (SERPS). To give webmasters the option to specify that the ODP content should not be used for listings of their website, Microsoft introduced in May 2006 the new "NOODP" value for the "robots" element of the meta tags.[9] Google followed in July 2006[10] and Yahoo! in October 2006.[11]

The syntax is the same for all search engines who support the tag.

Webmasters can decide if they want to disallow the use of their ODP listing on a per search engine basis

Google:

Yahoo!

MSN and Live Search:

If you add the NOYDIR tag to a page, Yahoo! won't display the Yahoo! Directory titles and abstracts.

Yahoo! also introduced in May 2007 the attribute value: class="robots-nocontent".[13] This is not a meta tag, but an attribute and value, which can be used throughout Web page tags where needed. Content of the page where this attribute is being used will be ignored by the Yahoo! crawler and not included in the search engine's index.

Examples for the use of the robots-nocontent tag: