This is a question that has worried human beings for thousands of years (well some of them anyway!)
If you draw any circle and then measure all the way round the outside (the circumference) and then if you measure straight across the middle (the diameter) then you will find that the first answer is always just a little bit more than 3 times the second.
People started to notice this thousands of years ago and tried to work out what the actual number is. Initially they used 3, then when they found this always gave an answer that was a bit small they used 22/7.
Even this is quite right though. The actual answer is a number we call pi. It has a name because we can't actually write it down exactly as a fraction or a decimal, it is an irrational number. But it is defined to be:
For any circle: the circumference / the diameter = pi.
This is quite useful because it means that if you know the diameter you can find the circumference by multiplying by pi, or find the diameter by dividing the circumference by pi.
It is also used to find the area of a circle which is found by multiplying pi by the radius squared.