It substantially depends on context. Different countries/regions/sates/provinces employ differing terminology and definitions. There are many terms in English to describe human settlements: village, town, township, borough, city, municipality, metro, urban area, conurbation...
As a general rule, yes, the word "city" usually refers to a place with a larger population than the word "town."
But there are many exceptions: It's certainly possible (in the USA, at least) for a "town" to have a larger population than a "city." In my home state (Massachusetts), for instance, the town of Brookline (63k) has a larger population than the city of Salem (44k). So, it's complicated, and can often be pretty arbitrary. Hope that helps!