This is a difficult question, because there is no precise answer. You would generally use "town" for places that have a population of roughly 50,000 or less but there is really no set rule. Some places with a smaller population like to call themselves "cities" and sometimes, in casual conversation, even large cities are referred to as "towns". Especially if you live in the countryside, you might say to your friend either "I'm going into town" or "I'm going into the city".

In Finland when using English town is generally called city if it has at least 100 000 residents... 8 big enough towns for that. But there's no clear rule.

In the United States, a city is defined by government structure, schools and the presence of a certain public services (e.g., library of a certain number of books, sanitation, transportation, government aid). It has nothing to do with the presence or absence of a cathedral in the States.

Officially a "city" is a town WITH a cathedral. If it doesn't have a cathedral it is still a "town". Giraffe is right it seems to depend on the size - but then usually the bigger urban areas also have a cathedral. ;)