What exactly is a biscuit?

In Australia we call cookies biscuits but I know in America its like a small side accompaniment to a meal. I used to think it was like a small roll, but then someone told me online that it was not like bread but more like a pastry or lump of pizza base type thing, containing alot of fat. Then today I saw a video on YouTube & the guy called the kfc bun a biscuit! Now I'm confused, because a kfc bun is a small version of their burger buns which is bread!