When you divide the circumference of a circle by the diameter of a circle, the number you get has an infinite number of decimal places. Numbers that do this are called "irrational".
Since this number is particularly important, we use this symbol (?) or just call it "pi" rather than expressing it with regular numerals.
The Bible once mistakenly referred to Pi as being just 3, with no decimals. Even at the time that passage was written, that was a grossly inaccurate estimate.
This is the longest estimate we've ever had of how long pi is. That number would be 310 million pages long and would take about 2.4 terabytes of hard drive space to store on a computer. That means that one number could take up more space than a 1/2 million mp3s.
(sources: 1, 2, ToothpasteforDinner)