Simply put one type of runners do it for fun exclusively. The other type of runners train to perform the best they can.
Obviously there can be subcategories of these groups based on age, speed, gendern so on...but simply put there really are only two off the bat.
Those who train with a purpose to be the best they can be and those who just run simply to run. At anytime one can move between the two groups once they've altered their purpose of the run.