cottonwood: any of several North American trees of the genus Populus having a tuft of cottony hairs on the seed
poplar: any of numerous trees of north temperate regions having light soft wood and flowers borne in catkins
relationship: a relation between people; (`relationship' is often used where `relation' would serve, as in `the relationship between inflation and unemployment', but the preferred usage of `relationship' is for human relations or states of relatedness)