Here's the breakdown:
* Bison are the large, shaggy mammals native to North America. They're the ones you usually see in pictures and imagine when you think of "buffalo."
* Buffalo are a different species of animal entirely, native to Africa and Asia. They're smaller than bison and have a hump on their back.
So, while "buffalo" is often used colloquially to refer to bison in North America, technically it's incorrect.
Here's an analogy: Calling a "dog" a "cat" because they both have fur and four legs. They might share some superficial similarities, but they're distinct species.
Think of it this way: Bison are like the "American Buffalo" and Buffalo are like the "African Buffalo."