You've probably heard of light, medium, and dark roasts, but what does it all mean? (No, dark roast coffee doesn't have more caffeine!) The most obvious difference between a light roast and a dark roast is visual: dark roasted beans cook for longer inside a roaster and therefore reach a darker brown color. But different roast levels do have implications for a coffee's flavor as well.
Roasting begins when coffee beans are dropped into the drum of a hot roaster. After several minutes of heating up, the water trapped inside the beans vaporizes and explodes outwards, causing the beans to pop loudly (like popcorn). This event is called "first crack".
After first crack, the beans start to transform rapidly due to a series of chemical reactions. You must carefully monitor the beans over the next few minutes and remove them from the roaster immediately when they reach the desired roast level. The images below should give you a feel for how different the beans can look at various points in the roast.
If you stop the roast right around first crack, you'll have a light roast. Light roast coffees tend to have more kick to them, a quality often called "brightness" or "acidity" in the coffee world.
Continuing the roast further will eventually result in a medium roast. Coffee roasted to this level starts to develop a pleasing texture or thickness, also referred to as "body." Medium roasts strike a balance between brightness and body.
If temperatures continue to climb, a second popping sound will start to occur. This is known as "second crack" (very creative names) and it heralds the arrival of the dark roast. At this point, brightness takes a backseat to fully developed body. The beans also develop an oily sheen and a stronger aroma.
If you continue too long past this point, you'll have a smoldering charcoal roast. This roast is known for its smoky aroma, toasty appearance, and carcinogenic aftertaste.
It is possible to go past a dark roast without totally ruining the coffee, and in fact some people prefer it. But you're treading on thin ice when you go that dark, and you'll want to bring a fire extinguisher just in case.
So which roast is best?
Like any question worth asking, there's no real answer. There is an argument to be made that lighter roasts better preserve the unique characteristics of the beans. In that regard, a dark roast could be compared to a well-done steak, in that the flavor of the bean itself is lost and you can only taste "the roast" instead. Some say it's easy to hide low quality or defects in the beans by roasting them on the darker side.
Ultimately, these are just opinions. You just have to trust your own taste buds. Don't let anyone, ourselves included, tell you what you can and can't enjoy.