Leaders - These are the likes of Google, Facebook, Microsoft, Yandex, Amazon etc. they develop ML libraries in languages like Python/C and export it to the world. They also read the latest research papers, extend new research, and push the frontier of ML. Working here is quite challenging because you need to be well versed with the theory of ML and also have good programming and software engineering skills. Followers - These are companies that use analytics to solve real life problems for e.g. in finance, loan prediction, or self driving cars, or retail analytics, or insurance propensity modelling, music recommendation etc. for these kind of roles, the emphasis is more on integrating the ML part with the product. You don't need to write a classifier code from scratch, but use existing libraries that the Leader companies create.