When the order doesn't matter, it is a Combination.
When the order does matter it is a Permutation.
"My fruit salad is a combination of apples, grapes and bananas" We don't care what order the fruits are in, they could also be "bananas, grapes and apples" or "grapes, apples and bananas", its the same fruit salad.
This is how lotteries work. The numbers are drawn one at a time, and if we have the lucky numbers (no matter what order) we win!
No Repetition: such as lottery numbers (2,14,15,27,30,33)
Number of combinations
where n is the number of things to choose from, and we choose r of them,
no repetition, order doesn't matter.
It is often called "n choose r" (such as "16 choose 3"). And is also known as the Binomial Coefficient.
Repetition is Allowed: such as coins in your pocket (5,5,5,10,10)
Or let us say there are five flavours of ice cream:
banana, chocolate, lemon, strawberry and vanilla.
We can have three scoops. How many variations will there be?
Let's use letters for the flavours: {b, c, l, s, v}.
Example selections include:
{c, c, c}(3 scoops of chocolate){b, l, v}(one each of banana, lemon and vanilla){b, v, v}(one of banana, two of vanilla)
Number of combinations
Where n is the number of things to choose from, and we
choose r of them. Repetition allowed,
order doesn't matter.
Permutations cheat sheet
Combinations cheat sheet
Permutations/combinations algorithm ideas.