The formula for choosing "n" objects out of "k" objects where you chose each object only once is: 

				k!
			-----------------
			    n! (k-n)!
where k! (pronounced "k factorial") is 1*2*3*...*k. To compute an easy example, if you had a lottery with 6 numbers and you had to choose 3 of them to win, the number of possible combinations would be: 

				6!
 			----------------
			    3!  (6-3)!

or

			1 * 2 * 3 * 4 * 5 * 6
		----------------------------------
		       (1 * 2 * 3)  (1 * 2 * 3)

or

			4 * 5 * 6 	
		-----------------------
			    6

which is 20.
Oddly enough this ties into Pascal's Triangle: 
	
				1
			      1   1
                            1   2   1
                          1   3   3   1
                        1   4   6    4  1
		      1   5   10  10   5  1
		    1	6   15  20   15  6  1
etc.
To find the number of ways you can choose 3 objects out of 6, you first count down to row 6 (noting that the first row is 0) and then count across to column 3 (again remembering that the first column is 0). Sure enough you get 20 again. Pretty hip huh? :)
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