There are two differences between honed and leathered granite: leathered finish adds texture to the surface, and it closes the pores making it more stain-resistant. Additionally, the color of the leathered finish looks darker and richer (though this may depend on the granite type)
This is a good description:
http://graniteimports.net/blog/leathered-granite-from-granite-imports-adding-greater-dimension-to-your-granite-countertops-in-denver/