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.
 A: ...or possibly double factorial $n!! = n \times (n-2) \times (n-4) \times \cdots $, or maybe subfactorial, $!n$, also known as derangement number. 
Added ... and superfactorial for desert.
A: For completeness:
Although in mathematics the $!$ almost always refers to the factorial function, you often see it in quasi-mathematical contexts with a different meaning.
For example, in many programming languages it is used to mean negation, for example in Java the expression !true evaluates to false. It is also commonly used to express inequality, for example in the expression 1 != 2, read as '1 is not equal to 2'.
It is also used in some functional languages to denote a function that modifies its input, as in the function set! in Scheme, which sets its first argument to the value of its second argument.
A: 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.
A: Probably you're thinking about the symbol for the factorial function:
$$n! = 1\times 2 \times 3\times \cdots \times (n-1) \times n$$
A: An exclamation mark can also be shorthand for "unique". For example, the statement
$$\forall y\in f(A), \exists!x\in A \text{ s.t. } y=f(x)$$
would be read as "for all $y$ in the range of $f$, there exits a unique $x$ in the domain such that $y=f(x)$". In other words, $f$ is one-to-one.
A: If it is $6!$, it would be $6 \times 5 \times 4 \times 3 \times 2 \times 1$, if it was $9!$, it would be $9 \times 8 \times 7 \times 6 \times 5 \times 4 \times 3 \times 2 \times 1$. Do you see the pattern?
$$6! = 720\\
9!= 362,880.$$
