observer vs attender

observer and attender both are nouns.

Word	Noun	Adjective	Verb	Adverb
observer	Yes	No	No	No
attender	Yes	No	No	No

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

observer: a person who becomes aware (of things or events) through the senses
attender: someone who listens attentively

observer (noun)	attender (noun)
a person who becomes aware (of things or events) through the senses	someone who waits on or tends to or attends to the needs of another
an expert who observes and comments on something	a person who is present and participates in a meeting
	someone who listens attentively