This interesting succulent is two species of cacti grafted together.  The top cactus is brightly colored in pink, red or yellow.   Top cactus is a "mutant pup"  and all nutrients are provided by bottom cactus referred to as the root stock plant.  Wide variety of species can be grafted together.  Average mature size is just 2"-6" tall and 1"-2" wide so this is another perfect plant for a desk corner.  This does not like direct sun, so stay away from the windowsill.