suppose in a joomla extension development, you need the host address. more clearly base uri. in this case you can use JURI::base() .
suppose you have a new jquery file that is stored in /media/system/js. you want to load the jquery in your view file (i am talking about joomla mvc development).
you can get this by the following code,
here i used JURI::base() to get the base uri.
hope that helps.