i = Unreal numbers. Like square root of -1.
TT (the other symbol) = Greek letter Pi, TT = 3.1415...
Irrational numbers are those that can't be expressed as fraction. i.e: 1/3 = 0.3333.. so it is a rational number.
You can't write TT like a fraction.
Did I explain myself??
Read that, it will be clearer :P hehe
http://en.wikipedia.org/wiki/Pi