I'm learning about interest (simple and compound interest). I don't understand it. What is interest exactly? I know the formula for simple interest is I=prt and you have to add the interest to the original amount.

The formula I learned for compound interest is
M = p (1 + i)ⁿ
But when I searched it up, it said it was
A = p (1 + r/n)ⁿᵗ

How do you know when to use which formula?

Can someone please clear up my confusion asap? I have a test and assignment due soon.