Short grain rice cooks up soft and tender, and is known for sticking together and clumping.
the harold mcgee type answer would mention that there are actually three grades: short, medium & long grain. while the shape does play a factor in how it cooks, it’s also about the level of starch (i want to say amylopectin but it’s been a while so i’d have to look it up. i looked it up - that’s actually the type of glucose which makes up the starch - my bad) in the grain, with medium & short grain rices having a higher starch content. for dishes such as paella & risotto, the best results occur with a medium grain rice such as arborio. it takes longer for moisture to penetrate shorter grained rices which is why it’s often suggested to soak those rices in water for at least a half hour before cooking to achieve the desired consistency. technique for cooking rice may seem quite trivial but someone no less than chef yasuda claimed that good sushi was 90% about the rice and how it’s prepared. little details matter.