A few versions back we added a DisplayName parameter to the user profile, so that users could give their preferred name. This was added after a number of community requests for an alternative value to display user information. Before modules and core code displayed either the username (e.g. "cathal)" or else showed the Firstname and Surname fields (e.g. "cathal connolly"). However, in some cases neither of these are ideal, such as cultures where the users name takes the form Surname, Firstname. Whilst this solved a common problem, not all sites want to use the display name value as the users can change the value potentially impersonating other users. To support both use cases, in 4.5 the displayname field supports tokens which can be used to build it's format.
To change the display name format, log in as an admin or host (superuser) account, and go to the Admin->User accounts menu option. Click on 'user settings', and enter your required settings in the Display Name format field. There are currently four tokens that you can use, [USERID],[FIRSTNAME],[LASTNAME] and [USERNAME], so if I wanted to show displaynames in the format "cathal AKA connolly,cathal", you would use [USERNAME] AKA [LASTNAME],[FIRSTNAME].