The difference between 여자 and 여성 is Hanja. To translate it into English... 여자 is for 'female human' = woman. 여성 is for female gender = woman. So 여성 is more used in a formal public situations like reports, articles, anouncements, presentation etc. 여자 is more used in personal situations like introduction or in normal conversations. Hope it could be a help. ;)