A pie is a warm baked dish made from a pastry dough.
If you meant Pi, Pi is the number used to define various measures in circles and spheres.
Some examples:
circumference of a circle =$${\mathtt{\pi}}{\mathtt{\,\times\,}}{\mathtt{diameter}}$$
area of a circle = $${\mathtt{\pi}}{\mathtt{\,\times\,}}{{\mathtt{radius}}}^{{\mathtt{2}}}$$
volume of a sphere = $${\frac{{\mathtt{4}}}{{\mathtt{3}}}}{\mathtt{\,\times\,}}{\mathtt{\pi}}{\mathtt{\,\times\,}}{{\mathtt{radius}}}^{{\mathtt{3}}}$$
area of a sphere = $${\mathtt{4}}{\mathtt{\,\times\,}}{\mathtt{\pi}}{\mathtt{\,\times\,}}{{\mathtt{radius}}}^{{\mathtt{2}}}$$
Pi is an irrational number, meaning you can't write it's exact value as it has an infinite amount of decimals.
Most people just use the first two to five decimals (3.14159), if you want to be thorough you might use the first 100 digits:
3.1415926535897932384626433832795028841971693993751058209749445923078164062862089986280348253421170679
If you feel like being very thorough:
http://www.geom.uiuc.edu/~huberty/math5337/groupe/digits.html (The first 100.000 digits of Pi)