We know html document consist of tags. Some of these HTML tags have no control over display of the page and they only used for search engines or for different robots who index the page. One of Such tag is meta tag. The most popular two Meta tags are Description Meta tag and Keyword meta tag.
Description meta tag contains a full sentence to give the best description about the page. Google insist through its webmaster tool about the minimum and maximum length of the description tag.
Keyword Meta tag consist of a set of different keywords used within the page. Webmasters use this tag to tell engines to rank this page for these set of keywords. However this tag is no longer considered by modern engines in their ranking but there is nothing wrong in having them. Let us start with some examples.
Description meta tag
<META NAME="DESCRIPTION" CONTENT="HTML tags and how to use them in your web page for formatting and SEO issues">
Keyword Meta tag
<META NAME="KEYWORDS" CONTENT="HTML tutorial, HTML tags, format page, tags for web page, web tags, bold tag">
There can be some specific meta tags like author name , version name of the page etc but they are ignored by engines. Here are some examples.