Short and nimble, halflings are predisposed towards a gregarious life. Their small posture has kept them hidden from the world in many places. Communities consisting of only halflings sometimes manage to stay forgotten for generations. In other areas, such as in the Sa'ar Desert, they are fully integrated into the general culture.
LightfootThe different kinds of halflings don't vary that much from each other. Lightfoots are the ones who have travelled the farthest and who are the most interested in interacting with the world outside purely halfling communities. They have inherited more wanderlust than their stout siblings. But they aren't always prepared to deal with everything in the world and have developed an innate sense of how to hide.
StoutStout halflings are very similar to their lightfoot cousins. The big difference is that the stout halflings are the descendants of those who have chosen a more stable life where they learn to deal with potential dangers instead of moving on. This has given them an ability to withstand poison and other such problems better than other halflings.
Genetics and Reproduction
Pregnancy and Childbirth. Halflings are pregnant for ten months, but have trouble getting pregnant for two years after giving birth. Both pregnancy and childbirth are typically easier for them than for other races.
Perception and Sensory Capabilities
While halflings have a lot in common with humans one thing, in particular, stands out: they have an almost supernatural amount of luck. Some theorise that Tyche herself favours them as her people. Less generous individuals grumble something about Asmodeus, but it's hard to see halflings as a whole race connected to the Nine Hells.