What on earth is wicked!?

last night i went to my friends pool birthday party, she got a purse that could play the music on her ipod. well, everyone said play wicked play wicked! so then she played some song called popular, what is wicked? i was the only one who didn't sing along and know what it was i kind of felt left out. they said it was a musical from Broadway, but i still have no idea what it is, please help :)