The metadescription is not visible in any part of your site, except if you check the actual HTML code. If set up correctly it is used by search engines as the snippet found in the SERP pages to describe a search result:
Actually, the metadescription is NOT used as a factor weighing in the search rankings, which sometimes causes people to think it is not important from an SEO perspective. However, their impact on Click Through Rates can be massive. The best way to describe it's importance is that you can see it as a little advertisement for your website, for free... Similar to the page title, attractive metadescriptions are very important.
The only thing Google hates is duplicate metadescriptions. They even stated on their webmaster blog that you are better off having empty metadescriptions then duplicate ones. If you do not enter the meta description for your pages, Google will just try and take a snippet from the text of your page. Usually it will do a pretty good job in doing so, except that it will usually look a bit messy, with sentences broken off, and the use of '....'.
However, if you enter the meta-description your self, you have full control of this text, as long as it is relevant for your search results. If a users searches for the word "SEO", but the metadescription you filled in does not contain this word, Google will probably ignore your description and use it's own. Using a correct meta-description has the following advantages:
- If you make sure you limit the description to 160 characters, it will not be broken off, and won't look messy.
- If carefully composed, users browsing through the search results may be more likely to click you site, increasing the Click Through Rate (CTR), which in itself can be positive for your SEO ranking.
- Google does NOT use metadescriptions to rank you higher. However, if set-up correctly, Google might register the higher CTR and reward you for this.
- Google hates duplicate metadescriptions. That could mean that it is better to have no metadescription at all then one metadescription for all pages on your site. This is a topic of much debate in the SEO world....
Setting a global meta-description in Joomla
When building a Joomla site, the first thing you are often advised to do is to add a global meta-description, using the Global Configuration settings. This ensures that every page will have a meta-description. However, it will be the same for all pages of your site, which is why I personally leave it empty (I add it for individual pages later). If you want to use it though, you can enable it from:
System > Global Configuration > Site tab, under Meta-Data
In the Site Meta Description field, fill in a usefull description, using your main keywords:
Meta-description for every page
You should however make sure that every page will have it's unique description. Then every page can build it's own ranking. In some cases, you might even manage to get several pages of your site listed in a Google search page.
You can set your descriptions on either article, category or menu-level. If you have a simple site with only a few articles, where every article is linked to a menu-item, it does not really matter which one you choose. On larger sites, or sites using blog-views, not every page is connected to a menu-item. Probably the best way to go is to set it in every article, for category views per category, and for the remaining ones on menu-level.
The most used method is through the Article Manager:
You can also choose to set metadescriptions from the menu-items. If both are entered, the menu-item description will be used.
Again, there are a lot of Joomla extensions that can help you with this process. In the JED you will find extensions that can even automate the process of generating meta-data for you: http://extensions.joomla.org/extensions/site-management/seo-a-metadata/meta-data. These extensions could work fine, but you have less control over the exact descriptions used. Often, they just pick the first 160 characters of an article. An example of a popular one that works quite well is Easy Frontend SEO.
Other extensions provide a clean overview for updating this information instead of having to go into each separate article every time. Most of the better SEF extensions like SH404SEF, MijoSEF, JoomAce etc. have this feature built in, but there are also extensions that provide just this functionality. One extension that I can highly recommend is OSmeta. It is easy to use, and gives you a perfect overview.
Pro-tip: It is always wise to perform an SEO audit that also scans whether your metadescriptions are indeed set-up correctly. I can highly recommend Onpage.org for this. It's free for one website (read my review of this service).