They are very similar and their uses inter-laps a lot, but here's an idea of when to use each: Relationship: It's between one person & another, or a small group. It has more sense of intimacy and familiarity. Jane and Jake have such a good relationship, I think they will get married soon. Our families have a strong relationship with each other. Relation: has less sense of intimacy and familiarity, it lacks a human emotion or it just feels abstract. Or very large groups (countries, big companies) The relation between negative and positive components of the atom show magnetic features. The relations between The US and The Middle East showing good progress.

read it thoroughly relation 7 b pl (1) : DEALINGS, INTERCOURSE {foreign relations} (2) SEXUAL INTERCOURSE relationship 3 a : a state of affairs existing between those having relations or dealings {had a good relationship with his family} b : a romantic or passionate attachment The chief difference between "dealings, [nonsexual] intercourse" on the one hand and "a state of affairs existing between those having relations or dealings" on the other is that the former (relations) refers to particular interactions, while the latter (relationship) deals with the overall state of affairs between the interacting parties.