For search engines that support them, META tags are used in at least three ways:
They are used to describe your site in the search results. For example, if you do not have a META Description tag for your page, some search engines will simply grab the first few lines of your page to serve as the description of your site. This description is more important than you may think. If you were searching for "nosuchwordasthis", which of the following two items listed would you be more likely to click?
If you do not put a description tag, what appears in the listing would be likely to be whatever first appears on your page, usually the alt text of some graphic or banner or perhaps your top menu.
There are search engines which also look for a META Robots tag to determine how they index your site. For example, if Google does not encounter such a tag, it will assume that it can index and follow the links on the page. If you want some other behaviour from Google, you will have to use a META tag to specify it.
In the old days, they were used to determine which keywords your page will be listed under. So if you want your page to be listed in a search engine when the user searches for the term "nosuchwordasthis", the latter should be present in your META Keywords tag as well as in your document. I'm not sure however if any search engine still uses this tag today.
There are other META tags, which individual search engines recognize. However, the tags listed here are the main tags relevant to most search engines.
How to Use the META Description Tag
A META Description tag, for those of you who write your web pages using raw HTML code, looks like the following:
<meta name="description" content="put your page description here" />
Give a brief description of your page in this tag. Bear in mind that this description (or part of it) will be displayed in the search engine results so try to make sure you phrase it in such a way that the person searching can tell at a glance that he's found the correct page for his search. Minimize irrelevancies and put the essentials near the beginning of the description so that if the search engine only accepts the initial (say) 150 characters, the essential parts of your description will still be displayed.
Not all search engines use this tag. For example, Google normally only displays the text surrounding the matching keyword that the searcher used.
How to Use the META Robots Tag
Although this Meta tag is seldom mentioned in search engine promotion guides, it is important if you want certain search engines to crawl through your website using the links on the page you submit. Since adding it involves little additional effort, you might as well do it while you add the other tags.
In general, you will add the following line to your web page if you want the search engine to index your page and follow the links on it.
<meta name="robots" content="index,follow" />
In cases where you do not want it to index your page simply use "noindex" instead of "index". Likewise if you don't want the engine to crawl through the links on your page, use "nofollow" instead of "follow".
Most, if not all, search engines today recognize this tag.
Furthermore, if you do not want the search engine to keep a cache of your page on their servers, you can affix a "noarchive" value to the "content" attribute, such as in the example below:
<meta name="robots" content="index,follow,noarchive" />
Google, for example, will see the "noarchive" value and not keep a copy of your page in its cache.
