Search engines interpret URLs as part of their effort to understand the subject, authority, and relevance of the content tied to that URL.The kind of URLs generated by Joomla and other CMS applications used to confuse search engines because they offer little semantic content, and there is more than one possible URL that will display approximately the same page content. An option to generate "Search Engine Friendly" URLs in the Joomla core and through various third-part extensions was developed to address this problem.

That said, search engines have adjusted to dynamic URLs, and SEF functionality is no longer a significant aid to search engine optimization (SEO). SEF does offer some SEO benefit if words in the URL match keywords in the corresponding page's content and metadata. But for the most part, SEF just looks good and makes page addresses more memorable to people. "Human-readable URLs" might be a better classification for SEF.

SEF works by redirecting the still always valid normal URL to an aliased URL that's simpler and uses words and numbers that are meaningful to people. Multi-lingual translations are also possible. Even with automatic caching of SEF URLS, SEF takes up more of the server's resources and can cause performance problems in some cases. However there are some security benefits to SEF as well, and the leading third-party SEF extension, sh404SEF, has some security and SEO enhancements as part of its capabilities.

As of Joomla 1.5.8 there appears to be a problem with the core Joomla SEF and core RSS syndication module. If the "add suffix to URLs" option is turned on in the global configuration, the feed links generated by the syndication module will be incorrect and broken on all pages other than the home page/any page using the frontpage component. There is no problem if this option is turned off. It's a feature that adds .html suffixes at the end of the SEF generated URLS for pages.

SEF can be tricky with page redirects and modules that use them, like third-party login modules (E.g., Core Design's and Community Builder's). Avoid making changes to the login system if that's possible.