A metatag is a line of information which may be in the head part of an HTML document. There are different kinds of metatag but the most important are the keywords tag, the description tag and the robots tag. Other kind of tags, like the author tag, not only are useless, from the SEO point of view, but can damage your rank since they put in the top part of the page not content related information, whilst we saw that for a keyword the closer is to the top of the page, the better is.
The metatags are not displayed to the readers of the page, but they are analysed by the crawlers.
They used to be very important in the ranking scores but since they are easily faked they are no more. However it does not take much afford to use them and even if the boost to the ranking is minimal, it would be a shame missing the chance.
The most important tag is the description one. It looks in this way:
<META name="description" content="A search engine shows the content of this tag below the title of your site when it appears in the results.">
Even if Google and other search engines does not support it, you should choose an appealing description for the page and incorporate the main keyword of the page since these tag appears at the beginning of the page. For the search engines which still show the description under the title of the page in the result, it would be a pity being at the top of the list but not being clicked by the users just because the description looks uninteresting. So you must keep the description about 13-15 words long and try to hit the point of the page in the first few words, in case the search engine cut the description because it is too long.
The Keywords tag is supposed to include a list of keywords that are relevant to the page. Once more the major search engines will not take in account this indication but it is a way to emphasize the keywords of the page and even their more common misspellings. If it is not a good idea to include typos in the content of the page because that can waste the reputation of accuracy of the whole website, here in this tag they can be useful with no side effects.
Even it is easy and quick developing these two tags, there are free tools that can build them for us, like the one at http://www.apromotionguide.com/metagene.HTML.
The Robots tag looks in this way:
<META name="robots" content="parameters">
The word 'parameters' should be replaced with two valid commands to the spider. The possible commands are: index, follow, noindex, nofollow. In a similar way to the Robots.txt this parameters are used to indicate to the spider if the page has to be indexed and if the links of the page are supposed to be followed. Anyway not all the spiders support this tag and the Robots.txt file is a better alternative to the use of this tag.