Roxas, Ventus and Sora?

correct me if im wrong.. all i know is ven and vanitas fought in bbs and ven was beaten and ven's heart wouldn't return to him..but it merged with Sora's heart?? then sora in kh1 became a heartless which created roxas as his nobody and thats how they look alike?? well if you guys know additional information please tell.. and do correct me if im wrong.. Ty! =D