What does the exclamation mark do? I've seen this but never knew what it does. Can any one let me in on the details? Thanks.

I just wanted to add a common usage for it as well since that may be of more interest to you.
A common usage for the factorial is in permutations.
For example:
If there were 3 people in a race, how many possible ways could there be to arrange the ranking. 
The answer would be 3! (3 factorial) or 3 x 2 x 1 or 6 ways.
Listing them the brute force way:
1) A,B,C
2)A,C,B
3)B,A,C
4)B,C,A
5)C,A,B
6)C,B,A
This extends into a lot of other things but most commonly associated with statistics, I believe.
