Whats the difference between a chin implant and a genioplasty?

Same thing usually: Any operation on the chin to change its shape is called a genioplasty from the latin word for "chin." sometimes an implant is used to change the appearance and other times the bone is cut and reshaped without an implant. So a chin implant is always a genioplasty but not the other way around.

Genioplasty: Strictly speaking, a chin implant is a prosthesis (i.e. silicon) that is inserted into the chin for purposes of augmentation. A genioplasty is reshaping of the chin by making osteotomies and repositioning the bony chin into a more esthetic position. One of the main differences between a genioplasty vs. a chin implant, is that there is no insertion of a foreign body.