Humans are the most adaptable and ambitious creatures among the common races. They have very different tastes, morals and customs in the many different countries where they have settled. But when they settle, they stay: they build cities that last for centuries and great kingdoms that can last for centuries. An individual Human may have a relatively short lifespan (~80 years), but a Human nation or culture preserves traditions with origins far beyond the reach of a single Human memory. They live fully in the present - making them well suited to the adventurous life - but also plan for the future and strive to leave a lasting legacy. Individually and as a group, Humans are flexible opportunists and remain alert to changing political and social dynamics.
In most worlds, humans are the youngest of the common races, late to appear on the world stage, and short-lived compared to other races. Maybe it's because of their shorter lives that they strive to achieve as much as they can in the years they get. Or perhaps they feel they have something to prove to the older races, and so they build their mighty empires on the foundation of conquest and trade. Whatever drives them, humans are the innovators, the achievers and the pioneers of the world.