When you create a facebook account you get a URL which is the web address or location being used in order to access your facebook profile.But it is usually too long like this for example :
www.facebook.com/profile.phd?id=478259118.
So it is gonna be hard to write it down or give it to your friends so they access your account.
 But you can change it to your username or any thing you wont, like this:
 http://www.facebook.com/username
Warning: you can only change your URL once so if you want to change it choose wisely

2. Go to Account Settings and scroll down to Username and click Change.

3. Write the new URL you want a check if it is available (like shown in the picture) then write your password and click "save changes".