What is pi?

Pi is the number equal to the circumference divided by the diameter of a circle. It's an irrational number so most people just use 3.14 or 3.14159. Though many memorize a lot more (hundreds) of digits, nobody is stupid enough to try to multiply by that.

The circumference of a circle is πR2 (R=Radius) or just πD (D=diameter).
The area of a circle is πR2.

See A brief history of Pi for, well, the history of pi.