In English this functions as an adjective and a noun but in Japanese Bishounen and Bishoujo are only nouns.
Originally from the Japanese word: Bishounen Bi, sho, nen.
Each character meaning:
Beautiful small year; respectively, roughly meaning "pretty boy"
Pretty
girl is somewhat different: Bi sho jou.
Again each character meaning:
Beautiful small woman, respectively. Roughly meaning "pretty girl"