Usually just the size. Hamlet is small, village is bit bigger, town a bit bigger, city bigger still. Citys in the UK usually have a cathederal too, but the Queen can issue city status to any place irrespective of if it has a cathederal or not.

town - can be one in a Rural community, being much smaller in size a population than a City City - is a great Metropolis in a much Built up area, which is comprised of Industry, buildings and people in multiple dwelling houses.