Translation:Who are you?
They didn't drill it a lot, but it's the informal second person of the verb to be: "tú eres", you are.
You can find any particular word by going to vocabulary tab and using the search field. Then if you click on the word itself -- in blue, to the left -- you can get a whole page about it -- for "eres" (ser), it looks like this:http://www.duolingo.com/word/es/ser/Verb
And you can click on "practice word" to get more practice with that particular word and its various forms -- which is good when it's something important like the verb "to be"
Don't be sorrry, that's why we're all here bro.
basically the difference comes down to plural vs. singular. if you're talking about 1 person "who are you?" "who is he?" it's quién, if it's multiple people "who are they?" "who are the people over there?" or "who are you people* it's quiénes.
Also, keep in mind that basically, quién is "who" whereas quiénes is "whom".
So "Where did the students to whom you gave the test to earlier, take that test?" would get quiénes used which is obvious because it's plural (students).
I'd like to point out that although technically poder is what Nnamdi18 indicated, the sentence presented by RAMOSRAUL also has an implied politeness to it. A literal translation of "¿Puede decirme su nombre?" is "Can you tell me your name?", but due to the implied politeness I referred to above, I think a better translation which keeps the same tone is "May I ask your name?" Even though that clearly is not a literal translation, I feel it better conveys the same level of politeness.
It means he/she/it is able to. So the above question says Puede(can or are you able to) Usted (formal version of you/tú) decir (verb for to tell or to say) me (pronoun used as an indirect object to the verb decir; me)su (pronoun;your) nobre (name)? So...Can you tell me your name?
And I recommend that if your own translation makes no sense like "who you?", just think about how you could change it so that it makes sense. Then type it, even if you don't understand it. After that, you can look at the discussion (like this one) to see why the answer isn't "who you?".