The magic of pi(π) is that π is the magic number. Pi is the ratio of the circumference of a circle to its diameter. Pi is always the same number, no matter which circle you use to compute it.
For the sake of usefulness people often need to approximate pi. For many purposes you can use 3.14159, which is really pretty good, but if you want a better approximation you can use a computer to get it. Here's pi to many more digits: 3.14159265358979323846.
The area of a circle is pi times the square of the length of the radius, or "pi r squared":
A = pi*r^2