Roasting changes green coffee into the aromatic, brittle material used for brewing. Heat drives off moisture and develops the flavors associated with roasted coffee. The familiar categories of light, medium and dark offer a rough way to discuss the result, but they are not a single standardized promise across every roaster's range.
A lighter roast may leave a coffee's origin characteristics more prominent, while a darker roast can make the effects of roasting more noticeable. This is a tendency to explore, not a ranking in which one category automatically wins. An espresso label also does not identify a separate species of bean. It usually indicates how a roaster expects that coffee to work as a concentrated, pressure-brewed drink.
To compare roast styles, keep the brewing method and serving size consistent, buy modest quantities, and note the flavors you actually enjoy. Descriptions such as city, French or espresso can be useful within one supplier's range but less precise across different brands. Ask the roaster what the label means in its own lineup, particularly if you are trying to repeat a previous purchase.
The conversation
A good question can make a better brew.