Search Engine Friendly URLs/13/en
From Joomla! Documentation
What is the Alias value? And how does it get created?
Alias is listed beneath the Title field in Articles, Categories, Sections and Menu Items. Joomla! can automatically create the alias for you. An automatic alias begins with the title. All upper case letters are changed to lower case. Spaces and special characters not allowed in a URL; they are changed to dashes.