From Joomla! Documentation
Before you even begin creating your Joomla site it is important to carry out a keyword research project to identify what the core keywords will be for your site, and associated keywords which complement or are relevant to the core keywords.
The reason for this is that keywords should be at the centre of your website design - your site structure (content categories, product groupings and so forth), menu items, content articles and so on should all be optimised to make the best use of your core and associated keywords.
As a basic guideline, ask yourself (or your client) the following questions:
What are my top 20 keywords?
What other associated words and phrases are important?
What questions will people be asking which my website will answer?
What will be the most important pages on my website (this is not always the homepage, often these will be pages within the website which contain specific information)
The meta keyword tag found in the head of most Web documents has little effect for Google, Bing and many of the other larger search engines any more, however some search engines such as Yahoo do still make use of the meta tags as part of their algorithm - so again this is something you should still continue to provide.
While it is true that many search engines do not use the information, they do still read the information - so make sure that your tags are relevant and descriptive of the content. As a rule of thumb, your main keywords should feature in your title, meta description, meta keywords and within the first few paragraphs of the content. If they don't, then consider whether you need to include them as keywords.
Keywords must be entered separated by commas (for example, "cats, dogs, pets") and may be entered in upper or lower case. (For example, "CATS" will match "cats" or "Cats"). Keywords can be used in several ways:
To help Search Engines and other systems classify the content of the Article.
In combination with Banner tags, to display specific Banners based on the Article content. For example, say you have one Banner with an ad for dog products and another Banner for cat products. You can have your dog Banner display when a User is viewing a dog-related Article and your cat Banner display for a cat-related Article. To do this, you would:
Add the keywords "dog" and "cat" to the appropriate Articles.
Add the Tags "dog" and "cat" to the appropriate Banners in Banners: Edit.
Set the Banner module Parameter 'Search By Tags' to "Yes" in Site Modules: Banners.
For articles only, in combination with the 'Articles - Related' module, to display Articles that share at least one keyword in common. For example, if the current Article displayed has the keywords "cats, dogs, monkeys", any other Articles with at least one of these keywords will show in the 'Articles - Related' module.
Keyword density used to be the buzz word of the Search Engine Optimisation (SEO) world, however it has become less important since search engines moved their focus away from meta keywords in their ranking algorithms.
At a basic level, keyword density is a way of explaining the number of times a word is repeated in the readable content of a page. If you have ten readable words and one word is a keyword the density of that keyword would be 10%. If you have 100 words and one of them is a keyword you now have a density of 1%.
It is recommended that content pages should try to ensure that keyword density falls somewhere between 3.5-7% as this is considered to be 'readable' without appearing 'spammy'. There are many online tools which will analyse your website to provide estimated keyword densities for your pages.
Guidelines on keyword use
It is important to keep at the forefront of your mind that any content on your website should first and foremost be written for human beings, not for search engine 'bots'. When writing an article, for example, ensure that when you read the text back to yourself it makes grammatical sense to include the keywords you are using in the places they have been used. If it sounds strange or odd, or if you've repeated words un-necessarily, adjust your text.
Keywords can (and should) be used in your title, alias, URL, meta description, meta keywords and within the content itself, but you should not try to 'stuff' your content full of keywords - it should always be pass the 'readability' test above.
Content that does not make sense and/or is full of keywords for the sole purpose of raising the keyword density to a high level could cause your site to be penalised by search engines, which could result in a drop in search ranking position and potentially de-listing of the page in question.
It is actually quite to write an article that someone will enjoy reading or find usable at the same time as having a keyword density over 7%, so it should be obvious if your content falls into this category of potentially spammy material.
This section has a keyword density on the term “keyword” of 1.79% and on the term “keywords” of 2.05%. The combined density is approximately 3.8%. You can see how deliberate you would have to be to stuff a site greater than 7%!
How to add meta keywords to your site
Add your core keywords to your Global Configuration > Site > Global site meta keywords if you wish to use global metadata
Add specific meta keywords (which may include core keywords) to articles using the metadata information (make sure the keywords listed here are in the content of the article, title, alias and meta description also)
Adding the data does not harm your rankings in search engines, and may help you in Yahoo, and meta crawlers
Do not add more than 25 words in the meta data
Ensure that you separate words and phrases with a comma
Do not repeat keywords