How Google Uses Joomla Meta Title, Description and Keyword Tags

Any web page, including one generated by Joomla can have "meta tags" or "elements" in the header information. The one's that get the most focus are the Title tag, the Description tag and the Keyword tag. This is mainly because it's thought that these have the most influence on Search Engine Ranking (which is not entirely true).
There are other tags/elements such as robots or language that provoke less discussion that the title, description or keyword, but are still important.
Lets take each Tag in turn and see how they are used in Joomla.

Meta Tags and Search Engines

Back in the 90's, getting ranked in search engines could be as easy as manipulating the content of the title, description and keyword tags. Now, this is no longer true, with Search Engine companies realizing the potential for abuser and developing increasingly more sophisticated algorithms to figure out what a web page is about (and hence be found on). The Director of Research at Google, Monika Henziger, was quoted (in 2002) as saying, "Currently we don't trust metadata because we are afraid of being manipulated." The old days of keyword stuffing will now potentially get you penalized rather than ranked!
But (there is always a but), most SEO experts agree that the Meta Description and Keywords are very devalued for ranking, the Meta Title is still a very very important factor in showing up on search engines... critical in fact!

Title Meta Tag

• Importance - 5/5
• In a Nutshell - Critical! Spend 5 minutes pondering your article titles
• Description - The most important of on-page keyword elements, the page title should preferably employ the keyword term/phrase as the first word(s).
• How Joomla Does it - Pages that are single articles with use the article title as the title meta tag. Pages that are sections of categories are trickier, usually using the name of the section/category as default. You can modify it however, by changing the System Parameters of the menu item linking to that page.

Description Meta Tag

• Importance - 3/5
• In a Nutshell - Not super important for ranking, but can be useful for visibility. I usually just modify the first sentence of the article and stick that in.
• Description - Although not used for "rankings", engines sometimes uses it for the snippet that appears in a search engine result. This can help boost click-through rate, thus increasing the traffic from any ranking position..
• How Joomla Does it - Pages that are single articles with use the description field as the description meta tag. Pages that are sections of categories are again harder, sometimes combining the description of child articles.

Keywords Meta Tag

• Importance - 1/5
• In a Nutshell - Long since devalued.
• Description - A holder over from 90's SEO, you are supposed to put keywords from your article into this tag
• How Joomla Does it - Same as the description, Joomla takes the field from the Metadata information and places it as a tag. Joomla does use these to match up related artilces for the Related Article Module. If you are using that, fill this in.

For these three tags, Title needs careful attention, the keywords can be left blank, and the description is in the middle. The Description Tag is not helping ranking, its helping click through... maybe...

Where Does the Google Description Snippet Come From?

According to Google, the snippet comes from the Meta Description, but this isn't always true. It's generally thought that if the term being searched on is included in your description, then Google will use content from your description tag in the snippet. If the term isn't, then Google might use "the meta description, from the description from the site’s Open Directory listing, from the content of the page body, or even a combination of these."
This excellent analysis gives 5 myths of Google snippets:

1. The term snippet is synonymous with the search listing and therefore includes the title, URL, Cached link, etc. Not true. Google makes very clear the fact that the term snippet applies solely to the description — and therefore follows the title and precedes the URL and the Cached link.
2. Google always uses the meta description in the snippet if it’s defined. That is far from the case. As already mentioned, snippets are query-specific and so they are always changing. Even when your meta description includes the search term, there are no guarantees.
3. Meta descriptions help with rankings — not just the snippet. This is patently untrue. According to Google, “while accurate meta descriptions can improve clickthrough, they won’t affect your ranking within search results” (from this aforementioned post.)
4. The bolded keywords in the search listing are bolded because they affected the ranking. Nope. The bolding of keywords (KWIC) is solely for user experience purposes. The meta description, as already stated directly above, does not influence a page’s rankings.
5. The maximum length of a standard snippet is 160 characters. I’ve seen various SEO bloggers asserting max lengths of 150, 156, 160, 161, and 165. What’s the correct answer? As mentioned above, 156.

So, where possible, it's worth taking a second to put something relevant in the Metadata Information of an article, because its not going to effecvt your ranking, you shouldn't spend anywhere near as much time as your article title (and hence Title Tag) which does.
A Very Very Important Point!!
Whether one of your web pages is showing up in Google is only partially influenced by these on-page factors, in particular the Title Tag. It's only #4 on what are the most important Search Engine Ranking Factors (SEOMoz).

1. Keyword Focused Anchor Text from External Links
73% very high importance

2. External Link Popularity (quantity/quality of external links)
71% very high importance

3. Diversity of Link Sources (links from many unique root domains)
67% very high importance

4. Keyword Use Anywhere in the Title Tag
66% very high importance

5. Trustworthiness of the Domain Based on Link Distance from Trusted Domains (e.g. TrustRank, Domain mozTrust, etc.)
66% very high importance

Get Help Getting Ranked

If you haven't already, you should check out Joomlashack's Scribe for Joomla plugin. A great tool to analyze content in your article and see what Google thinks it's about. From there you can make changes to help Google think it's about the same thing you do!
Using Scribe for Joomla, I get a score of 100% on this article, and The terms description, meta, joomla, keyword, and google are emphasized in the page content and are considered Primary Keywords.