The first META tags created in 1993 were intended to provide metainformation to robots, and most META tags used by web authors today are intended to improve the responsiveness of search services to authors' needs.
Many search engines and robots implement custom META tags that are unsupported by their rivals. This page only lists META values known to be used by more than one search engine. Engine-specific tags are listed on a separate page.
The Description value of the name attribute is probably the most commonly-used (by authors and robots) META tag. It was devised in 1994 for use by the site-index.pl indexing script, and further popularized when AltaVista and Infoseek began using it in 1995. Description was one of two META tags whose syntax was formalized at the 1996 Distributed Indexing/Searching Workshop.
Description is meant for a human-readable abstract or summary of a web page's contents that can be displayed in search engine results. Several major engines (and a lot of minor ones) use Description in this manner.
While, in theory, a Description can be up to 1024 characters long, most search engines will cut off the displayed description somewhere between 100 and 200 characters.
Search engines and web directories known to use Description in their search results include Aeiwi, Alltheweb, Any Search Info, Altavista, Blizg, ExactSeek, Free, FyberSearch, Gigablast, Google (sometimes), ILSE (defunct), Inktomi (sometimes), Metaseeker (defunct), NetInsert, search.ch, SeekOn, SplatSearch, SurfGopher, Teoma, Thunderstone, TrueSearch, Walhello, Websquash, and Wotbox.
The Keywords META tag was the other tag devised for use by site-index.pl in 1994 and adopted by other engines in 1995. Keywords is used to provide a short list of the most important topics covered by a web page. Multiple terms may be separated by spaces or commas.
The Keywords header has fallen out of favor at the major search engines. When it was first instituted, it was relatively easy for a web page to rank high in search engine results for words that only appeared in a Keywords header. After a great many commercial webmasters abused this effect, many search engines stop giving credence to the Keywords header. Some don't use it at all. (On the other hand, one search engine, Aeiwi, uses nothing but the Keywords header to determine a site's position in its directory.)
Even among engines that still recommend the tag, there's a difference of opinion on how to use it. Inktomi, for example, recommends using Keywords to list synonyms of the most important words, while Google says the tag should repeat the most important words that already appear on the page. Therefore, Keywords that help in one engine may do nothing, or even be counter-effective, in another.
Multiple keywords in a Keywords tag may be separated by spaces or commas, with no apparent advantage to using one over the other. (Logically, commas might make more sense, since the required behavior for multiple Keywords tags would be to concatenate them with commas, but that's far too pedantic to take seriously as a suggestion.) A given keyword may be repeated within a Keywords tag, but some search engines may reduce the ranking of pages that repeat a keyword too often.
The Robots tag was devised by a group of search engine professionals who gathered at the 1996 Distributed Indexing/Searching Workshop, as a way for web authors to control whether or not their pages are indexed by search engines. Although it began as a fairly simple tag, syntax extensions by various search engines have complicated it, so the ROBOTS meta tag has its own page here at META Tag Snob.
The Author tag is used to record the unqualified (common) name of a web page's author. It is not used for e-mail addresses.
I used to consider the Author tag just one of the useless tags devised for authoring software in the 1990s, but there are currently three search engines (Hotbot, Gigablast, and NetInsert) are advising authors to include the Authors tag. It might mean something to them after all.