Meta tags optimization
They are information inserted into the "head" area of your web pages, those are meta tags needed for optimization. Other than the title tag (explained below), information in the head area of your web pages can't be seen by the user. Instead, meta information in this area is used to communicate information that a human visitor may not be concerned with.
Title tag of a page
Most search engines use the site’s title tag as the link and main title of the site’s listing on the search results page. It tells the spider what the page is about and has a heavy weight in ranking results. The title is one of the most important components as far as search engines are concerned. These tags are usually wasted because few sites bother placing useful keywords in them or on the contrary overstuff them.
Meta keywords optimization
Although quite important in the past, this meta tag isn't’t as important these days. Some search engines do use it, but many don’t. This tag allows you to provide additional text for search engines to index along with your body copy. The fact is that this tag has been misused for years and only a few crawlers support it now. From the major players on the market, only Teoma and Inktomi are using the keyword meta tag in their rank calculation.
This tag is part of your successful meta tags optimization. There are two ways for the search engines to use the description tags. They read and index the text in the tag. In many cases, they use the text verbatim in the search results page. Then if your Web page is returned in the search results page, the search engine grabs the text from the tag and places it under the text from the title tag so the searcher can read your description.
Google and other search engines that it feeds won’t use this tag, at least in most cases. But if Google does not find your keywords in your pages (It may find it through inbound links), it will display the meta description tag.
Anyway Google and the search engines it feeds will index this tag, and most of other search engines use it.
By default search engines will crawl all your pages following links, the meta robot tag lets you specify whether or not some file might be indexed. This is important for security issues for example, or if you want to protect your client's data.
Even more tags...
There are many other meta tags than those presented above. In most case they are completely useless for optimization - search engines won't support them. For example, the meta revisit tag is supposed to tell the spider to index the site and to come back after a certain period of time - it just doesn't work, if it would, SEO would be an easier job.
If you are familiar with the search engine optimization techniques you probably already know all about meta tags. For the others, the meta tags optimization is one of the multiple areas to work on for a good SEO campaign.