Example
5!=5*4*3*2*1=120
! represents factorial

That is a mathematics question and explanation is lengthy. It is mostly used in numerical series. Best thing is to go to a mathematics site and search in it. Here is a sample

factorials get BIG really fast and exceed the limits of 32-bit unsigned integer variables by 13!.

If you want to play with them use a datatype that can handle huge numbers, or switch to an extended precision mathematics library like GMP.

Or use Ruby or Python that do not have these narrow limits.

>Or use Ruby or Python that do not have these narrow limits.
The only difference being that the arbitrary precision types are built into the language instead of offered as a library. I always enjoy when someone goes into a random thread and suggests that people use their favorite languages for no good reason.