Any of the ways we can arrange things, where the order is important.
Example: You want to visit the homes of three friends Alex ("a"), Betty ("b") and Chandra ("c"), but haven't decided in what order. What choices do you have?
Answer: {a,b,c} {a,c,b} {b,a,c} {b,c,a} {c,a,b} {c,b,a}
When the order does not matter, such as in a fruit salad, it is a Combination.
See: Combination