Search Engine Friendly (SEF) URLs
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.