rhododendron vs shrub

rhododendron and shrub both are nouns.

Word	Noun	Adjective	Verb	Adverb
rhododendron	Yes	No	No	No
shrub	Yes	No	No	No

As nouns, shrub is a hypernym of rhododendron; that is, shrub is a word with a broader meaning than rhododendron:

rhododendron: any shrub of the genus Rhododendron: evergreen shrubs or small shrubby trees having leathery leaves and showy clusters of campanulate (bell-shaped) flowers
shrub: a low woody perennial plant usually having several major stems

Other hypernyms of rhododendron include bush.

rhododendron (noun)	shrub (noun)
any shrub of the genus Rhododendron: evergreen shrubs or small shrubby trees having leathery leaves and showy clusters of campanulate (bell-shaped) flowers	a low woody perennial plant usually having several major stems