Darker roasts can be from increased roast time, or roast temp. I can do a shorter darker roast (which can mean there isn't enough time in the development phases), and I could also do a much longer light roast (which can result in baking, very undesirable). In general, a dark roast will be longer than a light roast of the same coffee. Dark roasts generally bring out the oils in the coffee, the darker you go, the more oily they will look, with a French roast looking almost wet.
Roast levels are usually described, in order of lightest to darkest: city, city+, full city, full city+, Vienna, French (or some variation thereof). There are two major markers in roasting, 1st crack and 2nd crack. First crack would correspond to a cinnamon or city roast, inedible at the start of first crack but becoming edible toward the end (even if it's going to be way bright and sour), then there's a lull before 2nd crack starts, which would be a full city+, and french is way deep into 2nd crack, or after. If you see a french roast in progress, and smell it, you'll understand why I avoid it: it's burnt, just billowing smoke and acrid notes. Most of my roasts are between the tail end of first (for a good Ethiopia, lets say), to the start of 2nd (for the Blend 55). But I can play all kinds of games with temps, times, and airflow (using more or less conductive or convective heat), and heavily manipulate the outcome.