N is the number of items you have. R is the number of places you can put the items where order doesn't matter. ex. 50 people enter in a raffle, the first 3 people to have their names draw will win a prize. To find how many different combinations of people you would insert 50 for N and 3 for R.
My Permutation calculator released yesterday is for if order does matter. You would use it if the example above was changed so that the first person to have their name drawn won $40 the second person would win $30, and the third person would win $20.
