In Joomla 2.5, you can use the Joomla Language Manager to modify language variables. Language variables are default text that is displayed in Joomla forms, pages, and screens. For example, the field names displayed on the default Joomla Contact form (Full Name, Email, Subject, and Message) are stored in the com_contact language file. Furthermore, Joomla Extensions use language fiels to store language variables.
Of course you can edit a language file directly in /language/en-gb/ but you first have to find the correct language file. The Joomla Language manager makes it much easier to search for and modify the screen text.
Another advantage of using the Language Manager to create an override is that your language modification will not be overwritten when you upgrade Joomla or a Joomla Extension.
If you modify a language file, your changes will be overwritten when you upgrade Joomla.
For information on how to use the Joomla Language Manager to create language overrides, click here.