vs
QUICK ANSWER
"Quienes" is a form of "quienes", a pronoun which is often translated as "who". "A quién" is a phrase which is often translated as "to whom". Learn more about the difference between "a quién" and "quienes" below.
A phrase is a group of words commonly used together (e.g., once upon a time).
a. to whom
Si no sabe a quién dirigir la carta, llame y pregunte.If you don't know to whom you should address the letter, inquire about it over the phone.
A pronoun is a word that stands in for a noun (e.g., she).
1. (relative)
2. (indefinite)
a. whoever
Felicito a quienes hayan participado en el concurso.Congratulations to whoever participated in the contest.