Meta tags can be included in the HTML code of a web page and contain information to help and direct web Robots (those little programs sent out be search engines to index the web) to retrieve information about a page. There are already excellent resources on this subject on the web and this page includes several links you should follow.
Gone are the days when search engine robots indexed sites purely on their meta tags. Things are much more sophisticated now, they focus much more on the actual contents. Therefore many meta tags are a waste of time but a few are useful and one or 2 essential.
Short answer, no.
There are a number of meta tags available to the web designer however the majority of them are only useful in very limited circumstances and some (such as the refresh tag) can be harmful to your ranking. As a general rule of thumb unless you can think of a good reason to include a meta tag, don't bother.
There are of course certain exceptions, however most search engines ignore large amounts of meta tag data and Google has gone as far as saying it ignores all meta tag data (at least for ranking purposes).
The description meta tag is used by some search engines although probably not for ranking but as a short bit of text to display in their results page. Every page should have a unique description, using the same description on multiple pages may negativity effect your search engine ranking.
The keyword tag I believe can be chosen to be ignored. Any reputable search engine will use it's own methods to determine the content of the page. This is partly due to the misuse of meta tags in the past and partly to do with the fact that even well meaning web designers will often pick poor or incorrect keywords. Again if you do use keywords make sure each page is unique as using the same keywords on multiple pages may negativity effect your search engine ranking, it is probably a good idea to work under the general guideline of less is more in the case of keywords.
The robots meta tag should be used as an addition to robots.txt and .htacces as a method of controlling how a search engine indexes your site, if you are unsure of how this works it is best to leave it out. This tag can alter your search engine ranking as many search engines will place emphasis on which sites you link to. It is also worth noting that not all search engines actually do what you ask them to and some may ignore the robots meta tag.
This tag is created by Dreamweaver. You must declare content type to pass validation tests ...
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
The importance of meta tags to web spiders is sometimes over emphasised, but every little bit helps. By way of example, here are the meta tags used for our pages ...
<meta name="description" content="Free resources for multimedia music makers">
<meta name="keyword" content="Matt Ottewill, Don Kallenbach, Multimedia, web design, DTP, digital video, DVD authoring, sound engineering, sequencing, synthesis, MIDI, music technology, help, tutorials, advice">
<meta name="robots" content="index, nofollow"> --This tag tells a robot/spiders to index all content but not to follow external links
<meta name="copyright" content="© 2005, Matt Ottewill, Planet OF Tunes">
<meta name="author" content="Matt Ottewill unless otherwise stated">