watcher vs observer

watcher and observer both are nouns.

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

As nouns, observer is a hypernym of watcher; that is, observer is a word with a broader meaning than watcher:

watcher: a close observer; someone who looks at something (such as an exhibition of some kind)
observer: a person who becomes aware (of things or events) through the senses

Other hypernyms of watcher include beholder, perceiver, percipient.

watcher (noun)	observer (noun)
a guard who keeps watch	a person who becomes aware (of things or events) through the senses
a close observer; someone who looks at something (such as an exhibition of some kind)	an expert who observes and comments on something
a person who keeps a devotional vigil by a sick bed or by a dead body