n C r denotes the number of combinations of n things taking r at a time.

Steve,
  I did the same thing initially. If you choose 1000 of 2000 items, and the order that you choose the items matters then the equation is:

n! / r!. 

However, if the order does not matter, (1,2 is the same as 2,1) then the equation becomes:

n! / (r! * (n-r)!)