'Who are you ?' in spanish is 'quien eres tu?' instead of 'quien es usted ?'.. y ?
There's no 'd' in 'eres'.
Check out the conjugation of ser here.
tú eres - second person singular
usted es - third person singular formal
Both Quién es usted? and Quién eres tú? both mean Who are you?
As Jack said, the tú form is the informal you. You should use it among people you are really close to like friends and family. The usted form is the formal you. You should use it when you are trying to show respect or honor to someone, or if you just don't know them that well (ex. your teacher, boss, coworker, neighbor, a random stranger you just met, etc). I hope this helps.
is the familiar form of 'you' when you know someone well or if it is a smaller child or someone younger than you (if it is the first you wouldn't have to ask who they are).
is a more formal way to say 'you'.