phobia vs agoraphobia

phobia and agoraphobia both are nouns.

Word	Noun	Adjective	Verb	Adverb
phobia	Yes	No	No	No
agoraphobia	Yes	No	No	No

As nouns, agoraphobia is a hyponym of phobia; that is, agoraphobia is a word with a more specific, narrower meaning than phobia:

phobia: an anxiety disorder characterized by extreme and irrational fear of simple things or social situations
agoraphobia: a morbid fear of open spaces (as fear of being caught alone in some public place)

Other hyponyms of phobia include simple phobia, social phobia.

phobia (noun)	agoraphobia (noun)
an anxiety disorder characterized by extreme and irrational fear of simple things or social situations	a morbid fear of open spaces (as fear of being caught alone in some public place)