What is a ball of yarn?

[Knitcracker]

Hello there,

I was wondering if someone can help me here. I have been searching the internet to find out - What exactly is a ball of yarn? Patterns I see call for a ball of this and and / or 2 balls of that. What is this standard of measurement? I'm certain it's not a skein of yarn because I have just bought a skein thinking that's what I needed and had PLENTY left over for several of the same project.

Thank you for your time
KC