Mathematics is fascinating. Wait until you get into trig functions.
Pi is an irrational number. This means that it cannot be expressed as the quotient of any two whole numbers, and that it goes on forever, in no obvious pattern (3.14159265...)
"i" is called imaginary. It is defined as the square root of negative one (I'll be writing sqrt(-1) for this). Now, why would anyone call sqrt(-1) irrational? Well, think of what a square is: it is a number multiplied by itself. Now, can you think of any number that, when multiplied by itself, gives a negative number? Let's try: a positive times a positive = a positive. A positive times a negative = a negative. But that's not a number multiplied by itself, is it? 2 time -2 is not 2 times 2, right? And finally, any negative times a negative equals a positive.
So, how that's why we call i, which is sqrt(-1), "imaginary" - because according to our "common sense" there's no number that really can be the sqrt(-1).
I hope this helped.