When sampling r objects from a population of n objects you can...
1) Sample with replacement or without replacement
2) Sample with order (ordered) or without order (unordered)
 
NOTE: With Order means that sample '1 2' would be counted separately from sample  '2 1'
but Without Order means they would not be counted separately.
 
The table below summarizes the enumeration methods for the four cases.

	With Order	Without Order
With Replacement	nr	n-1+rCr
Without Replacement	nPr	nCr