If you've set your Joomla website to generate search engine friendly (SEF) URLs, each page will automatically generate an 'Alias'.
The 'Alias' can be manually configured if required to improve the chances of the page being found in a search. In other words think of what keywords might be used, and set the value of your 'Alias' accordingly.
Once an Alias has been configured, individual pages can then be found by inserting the appropriate URL in the search field of your browser. For instance if the 'Alias' for a page is set to 'domain-names', the correct URL to use would be: www.yourdomain.co.uk/domain-names.html
* please note that leaving the .html extension off will also work when entering a URL in your browser address field. In other words, entering www.yourdomain.co.uk/domain-names should work fine.
Make sure that when you use SEF URLs you ensure any pages (articles) are declared on a Menu, even if you don't want the Menu or the article to be seen. Not adding a page to a menu will stop the www.yourdomain.co.uk/domain-names.html style of URL from working, and looks as though it may give problems when declaring the pages on a sitemap.xml file, inasmuch as Google Webmaster issues a WARNING.