Character ID
Every in-game player character has a Character ID number. It is automatically assigned by the server upon character creation. It is unique and sequential, so you may get some idea of how old a character is based on his ID.
You can see any character's ID number. Alt-Click on a character and you'll see a short vertical menu. That character's ID is shown there at the top of the list.
When you Reroll a character, the original ID is left behind forever, and a new ID is automatically assigned by the system to the new character.