Given a good-quality bean, roasting is probably the single most important factor influencing the flavor of coffee. The most significant variable is degree, or darkness, of roast. The longer coffee is held in the roaster and/or the higher the roasting temperature the darker the bean. The darker the bean, the more tangy and bittersweet the flavor. When this flavor settles onto the uninitiated coffee drinker’s palate, the usual response is to call it strong.
However, strength in coffee properly refers to the proportion of coffee to water, not the flavor of the bean. The more coffee and the less water, the stronger the brew. So you could make a light-roasted, mild-flavored coffee very strong, and brew a dark-roasted, sharp-flavored coffee very weak.
I would rather call this dark-roasted flavor dark, pungent, bittersweet, or tangy. This flavor occurs in degrees, depending on how dark the bean is roasted and how the bean is roasted (quickly at high temperatures, slowly at lower, etc.). It peaks when the bean is roasted to a very dark brown, and eventually vanishes entirely to be replaced by a charred, carbon taste when the bean is roasted almost black. To understand the chemistry behind the changes in taste, we need to examine what happens when a coffee bean is roasted.