It's considered a light roast when the beans are pulled early in the first crack phase while the beans are still popping slightly. 

This generally produces higher acidity and a marginally higher caffeine content, which is why so-called "breakfast blends" tend to be lighter roasts and more acidic.

Coffee beans that are known for fruity and floral notes are preferred at a light roast because those characteristics often dissipate in longer roasting times. 

Examples of coffee beans are Geisha, Colombian and African coffees. 

For brewing, a pour over is ideal with purified water instead of tap. Light roasts aren't powerful enough to mask impurities in tap water. An automatic drip coffee maker is known for making it even more acidic.

You can make a delicious espresso with a light roast with the right preparation and technique. It requires a finer grind, higher extraction temperature and longer pull than a traditional dark roast.

It's considered a medium roast when the beans are pulled as the first crack is beginning to slow down. The beans may pop a little, but not as much as the light roast. 

This produces a smoother and more full-bodied coffee while still retaining much of its more delicate characteristics. 

Coffee beans that have notes of nuttiness or chocolate tend to handle darker roasts well, but fruit needs a lighter roast. The medium is ideal for beans that have more than one characteristic. 

Examples of coffee beans that do well as a medium roast are Papua New Guinea, Nicaragua, Mexico and Brazil. 

For brewing, medium roast can work with any hot method since it has the best of both worlds. Espresso will still require a little extra dialing in for preparation. Medium roasts work well as cold brew and iced coffee since they retain more natural sugars. 

It's considered a med-dark roast when the beans are pulled immediately before the 2nd crack with only a couple of pops as it's cooling. 

The purpose of a med-dark roast is to develop more caramelization of the natural sugars in the bean. This produces a velvety smooth, full-bodied coffee that chills well in the cup. 

Coffee that has strong notes of chocolate and nuttiness work best at this stage. By this point in the roasting process, any fruity notes will be gone. 

Examples of beans that do well at this roast is El Salvador, Guatemala, Peru, Honduras and some Mexican coffees. 

​For brewing, once again any hot method works beautifully. Espresso won't require too much extra preparation to dial it in. These coffees are amazing as cold brew and iced coffees as well.  

It's considered a dark roast when the beans are pulled during the 2nd crack with the beans popping full force as they're cooling. 

While there is a time and a place for full-on dark roasts, I tend to steer away from them and stick with the med-dark.

Commercially, such as Starbucks and Dunkin, the purpose of a dark roast is to cover the defects in the beans. If you roast a bean long enough, it tastes like everything else. 

On a more refined scale, such as your local bowtie-wearing Ba-Roasta, the purpose of a dark roast is to develop more sugars and produce a heavy-bodied coffee that works well as espresso. Some coffees have a brown sugar natural sweetness to it that becomes more like molasses at a darker level. Other coffees have funky characteristics or are just boring unless they get a heavier roast on them.

Examples of coffees that require a dark roast is Sumatra, Indian Monsoon Malabar and even in my experience there have been some Ethiopian coffees that required it. 

For brewing, espresso works pretty well because that's how most people take their espresso. For regular hot coffee, any method will work fine because the coffee flavor is powerful enough to cut through impurities in the water. Cold brew and iced coffee can be a little too bitter when served cold.