Real Dictionary

yes
convention: You use yes to give a positive response to a question.

yes in American English
aye; yea; it is so: the opposite of no, used to express agreement, consent, affirmation, or confirmation
adverb: (used to express affirmation or assent or to mark the addition of something emphasizing and amplifying a previous statement)
interjection: (used as a strong expression of joy, pleasure, or approval)
noun: an affirmative reply
transitive verb: to give an affirmative reply to; give assent or approval to

yes in British English
noun: an answer or vote of yes
sentence substitute: used to express acknowledgment, affirmation, consent, agreement, or approval or to answer when one is addressed