META tag used to help define the primary keywords of a Web page.
Initially, the META keywords tag (keywords tag for short) was used to provide additional information about a page (metadata). It sounded good in theory. In practice, the hidden META tags were often used in ways they were never intended, including the addition of excessive or irrelevant keywords. Now, instead of providing additional information, the META tags are often used to reinforce content that is already on a page.
The growing trend towards off-the-page criteria seems to have lessened the importance of the META keywords tag.
References to this tag vary. Some use keyword tag (singular) and some use keywords tag (plural). The actual tag uses “keywords” to describe itself, so the plural would seem to be a correct usage.
The keywords tag belongs in the <HEAD> section of a Web page, typically placed below the META description tag. Do not use any keyword excessively, as search engines will perceive this as spamming. Also, keep the keywords and phrases relevant to the actual text of the page.
see also: SEO Tools