What are the usernames ?
Your Facebook username is also your Facebook email address. If till now you have not got your Facebook username then get it at http://www.facebook.com/username . I can give you an example. If your Facebook username is example123 then in the URL of your profile page it will appear as http://www.facebook.com/example123. And also your Facebook email ID is firstname.lastname@example.org. So you can also login using example123 as your username.
What are the passwords ?
It is your original password which you generally use to login your Facebook account.
Facebook does not care about the case of your 1st letter of your password. You can type it in either uppercase or lowercase. For example if your password is Example123 then you can write the first letter as either ‘e’ or ‘E’. That means you can also write it as example123. This facility will be really very useful in mobile devices.
Just convert the upper case letters of your password to lower case and vice-versa, you will get the 3rd password. For example if your password is Example123 then you can also write it as eXAMPLE123. This will be helpful if by mistake you have made the caps lock on.
(You may also like to see Some Interesting Facebook tricks)