After several minutes in the heat of the roaster, beans begin to make cracking or popping noises and they begin to expand in size-this is called first crack. This is typical of most mass-produced coffee in the US. The characteristics of this roast level are a lighter body, no "roast" flavor and a higher level of acidity. Note, the lighter the roast, the more caffeine in the coffee.
A few short minutes after first crack, the beans reach this level of roast, which is very popular among specialty coffee retailers. The characteristics of this roast level are a slight sweetness, more body, and increased aroma and complexity.
Another few minutes after hitting medium, the beans start to pop again, and oils in the beans come to their surface. This stage is called second crack. The characteristics of this roast level are slight spiciness, heavier body accompanied by a heavier mouth feel and at this stage the flavor and aroma of the roast are quite evident.