weeping willow vs willow tree

weeping willow and willow tree both are nouns.

Word	Noun	Adjective	Verb	Adverb
weeping willow	Yes	No	No	No
willow tree	Yes	No	No	No

As nouns, willow tree is a hypernym of weeping willow; that is, willow tree is a word with a broader meaning than weeping willow:

weeping willow: willow with long drooping branches and slender leaves native to China; widely cultivated as an ornamental
willow tree: any of numerous deciduous trees and shrubs of the genus Salix

Other hypernyms of weeping willow include willow.

weeping willow (noun)	willow tree (noun)
willow with long drooping branches and slender leaves native to China; widely cultivated as an ornamental	any of numerous deciduous trees and shrubs of the genus Salix