Semi Metallic brake pads consist of organic and metal materials. They are often mediocre brake pads which will perform pefectly fine when it comes to braking (Usually what OEM comes with). But the downfall of Semi-Metallic brake pads is that its extremely dusty compared to Ceramic brake pads. The Upside of it is that it has outstanding wear characteristics and is a very quiet brake pad.
Ceramic brake pads consist of ceramic compounds and copper fibers which allow ceramic pads to handle the high brake temperatures with less brake fade. Not only that ceramic pads has the least amount of brake fade, it also has the least amount of brake dust. Ceramic pads are made with a harder compound which may be a little bit more noiser than a softer compound such as semi-metallic, but the stopping power really out weighs using semi-metallic.