UPDATE 30.1.2015 Google+ Help Center articles renewed
Now the information in the official Help Center for Google+ users about the Custom URL's is separate for profiles and for business Pages.
When you have a custom URL, you can give people a short, easy-to-remember web address to find your Google+ profile (like google.com/+LarryPage). it will be preassigned based on things like your name. And you may also need to add a few letters or numbers to make it unique to you.
Important: You can’t change your custom URL after you create it, so be sure you like yours before you finalize it.