Given two sets, an inner join will combine the two sets into one where the result is based on the elements which are alike. An outer join combines two sets into one where the result is based on the elements that aren't present in both sets. An example of a inner join would be: {1, 2, 3, 4, 5} join {1, 3, 4, 6, 7} = {1, 3, 4} An example of a outer join would be: {1, 2, 3, 4, 5} join {1, 2, 3, 5, 6} = {4, 6}