簡單來說，relation 指的是「關係」，而 relationship 指的是「關係本身」。然而在 webster 字典裡，兩者的解譯完全一樣 (the way in which two or more people, groups, countries, etc., talk to, behave toward, and deal with each other)，必需從例句去理解其微妙的差異。
例如：The incident led to tense international relations. （不強調關係本身，而是強調產生了一種關係）We have a good working relationship. （強調關係本身是好的或壞的）
Others
relation 代表「親戚」，by marriage 就變「姻親」
relationship 還有「浪漫」的關係的意思：a romantic or sexual friendship between two people
Reference
https://english.stackexchange.com/questions/15208/relation-versus-relationship
https://forum.wordreference.com/threads/relation-relationship.29820/