Factorial !
Example: 4! is shorthand for 4 x 3 x 2 x 1
The factorial function (symbol: !) means to multiply a series of descending natural numbers. Examples:

Calculating From the Previous Value
We can easily calculate a factorial from the previous one:
n  n!  

1  1  1  1 
2  2 × 1  = 2 × 1!  = 2 
3  3 × 2 × 1  = 3 × 2!  = 6 
4  4 × 3 × 2 × 1  = 4 × 3!  = 24 
5  5 × 4 × 3 × 2 × 1  = 5 × 4!  = 120 
6  etc  etc 
 To work out 6!, multiply 6 by 120 to get 720
 To work out 7!, multiply 7 by 720 to get 5040
 And so on
Example: 9! equals 362,880. Try to calculate 10!
10! = 10 × 9!
10! = 10 × 362,880 = 3,628,800
So the rule is:
n! = n × (n−1)!
Which says
"the factorial of any number is that number times the factorial of (that number minus 1)"
So 10! = 10 × 9!, ... and 125! = 125 × 124!, etc.
What About "0!"
Zero Factorial is interesting ... it is generally agreed that 0! = 1.
It may seem funny that in this case multiplying no numbers together results in 1, but it helps simplify a lot of equations.
Where is Factorial Used?
Factorials are used in many areas of mathematics, but particularly in Combinations and Permutations
Example: What is 7! / 4!
Let us write them out in full:
7 × 6 × 5 × 4 × 3 × 2 × 1 
= 7 × 6 × 5 = 210 
4 × 3 × 2 × 1 
That was neat. The 4 × 3 × 2 × 1 "cancelled out", leaving only 7 × 6 × 5
A Small List
n  n! 

0  1 
1  1 
2  2 
3  6 
4  24 
5  120 
6  720 
7  5,040 
8  40,320 
9  362,880 
10  3,628,800 
11  39,916,800 
12  479,001,600 
13  6,227,020,800 
14  87,178,291,200 
15  1,307,674,368,000 
16  20,922,789,888,000 
17  355,687,428,096,000 
18  6,402,373,705,728,000 
19  121,645,100,408,832,000 
20  2,432,902,008,176,640,000 
21  51,090,942,171,709,440,000 
22  1,124,000,727,777,607,680,000 
23  25,852,016,738,884,976,640,000 
24  620,448,401,733,239,439,360,000 
25  15,511,210,043,330,985,984,000,000 
As you can see, it gets big quickly.
If you need more, try the Full Precision Calculator.
Interesting Facts
Six weeks is exactly 10! seconds (=3,628,800)
Here is why:
Seconds in 6 weeks:  60 × 60 × 24 × 7 × 6  
Factor some numbers:  (2 × 3 × 10) × (3 × 4 × 5) × (8 × 3) × 7 × 6  
Rearrange:  2 × 3 × 4 × 5 × 6 × 7 × 8 × 3 × 3 × 10  
Lastly 3×3=9:  2 × 3 × 4 × 5 × 6 × 7 × 8 × 9 × 10 
There are 52! ways to shuffle a deck of cards.
That is 8.0658175... × 10^{67}
Just shuffle a deck of cards and the chances are you are the first person ever to have that particular order.
There are about 60! atoms in the observable Universe.
60! is about 8.320987... × 10^{81} and the current estimates are between 10^{78} to 10^{82} atoms in the observable Universe.
70! is approximately 1.197857... x 10^{100}, which is just larger than a Googol (the digit 1 followed by one hundred zeros).
100! is approximately 9.3326215443944152681699238856 x 10^{157}
200! is approximately 7.8865786736479050355236321393 x 10^{374}
Advanced Topic
What About Decimals?
Can we have factorials for numbers like 0.5 or 3.217?
Yes we can! But we need to get into a subject called the "Gamma Function", which is beyond this simple page.
Half Factorial
But I can tell you the factorial of half (½) is half of the square root of pi = (½)√π, and so some "halfinteger" factorials are:
n  n! 

(½)!  √π 
(½)!  (½)√π 
(3/2)!  (3/4)√π 
(5/2)!  (15/8)√π 
It still follows the rule that "the factorial of any number is that number times the factorial of (1 smaller than that number)", because
(3/2)! = (3/2) × (1/2)!
(5/2)! = (5/2) × (3/2)!
Can you figure out what (7/2)! is?