Carroll is normally a man's name; Carol is normally a woman's name these days, but can be either.

Actually they have different origins - Carroll is an anglicised version of "Cerbhall", which in Irish means "Brave in battle"; Carol is a version of Charles, when it's a male name; but when it's a female name it probably is just a Christmassy sounding name from Christmas Carols. Though how and why it became a female name is a mystery. Except that pretty well any name that sounds pleasant is liable to be used as a woman's name sooner or later.