Me neither. It's not really a useful distinction as far as I can tell. What would you use it for?

Where did you read this and what was the context?

The difference is really in how much light they absorb and therefore how easily damaged by light they are. Think of it like getting a sunburn: you are more likely to burn your shoulders (horizontal) then your legs (vertical) because your shoulders get more direct sunlight. It's the same with plants: horizontal leaves are really good at absorbing light (especially in low light conditions) but are more easily damaged by the sun, while vertical leaves (like grass) function better in high amounts of sunlight and aren't as easily harmed by it. As for how to identify them, it's as simple as whether the leaves are horizontal or vertical.