Johannesburg vs city

Johannesburg and city both are nouns.

Word	Noun	Adjective	Verb	Adverb
Johannesburg	Yes	No	No	No
city	Yes	No	No	No

Johannesburg (noun)	city (noun)
city in the northeastern part of South Africa near Pretoria; commercial center for diamond and gold industries	people living in a large densely populated municipality
	a large and densely populated urban area; may include several independent administrative districts
	an incorporated administrative district established by state charter