*just an assistant ESL teacher
I couldn't think of the answer, but I found this: S.O.S. Mathematics CyberBoard :: View topic - Formula vs. Equation
And I came to the conclusion that an equation has an equal sign. We often see formulas expressed with an equal sign, but the formula is actually the rule for figuring something out. So when the area of a circle (A) = pi r squared (sorry, I don't know how to properly insert pi and squared), the "pi r squared" is the formula. I think.