When coffee beans are roasted the moisture in the beans expands causing the beans to crack open. Depending on the roast level beans go through either one or two "cracks". Beans from the same crop, roasted to different temperatures will taste different. Some generalizations of roast level on flavour:
Light: Also called City, Cinnamon or Blonde. Light brown in colour with no oil, the beans have just gone through the first crack. There is a toasted grain taste with pronounced acidity. They have a mild flavour that reflects the region they were grown.
Medium: Also called Full City or American, the beans have gone fully through the first crack and are approaching the second. The aroma and acidity is quite balanced in this cup with notes of fruit, spice, chocolate and caramel. Expect complex, nuanced flavours that ebb and flow as the cup temperature changes.
Dark: Also calledVienna, French, Italian,the beans have gone through first and second crack. Dark, oily brown in colour, expect smoky, earthy, chocolate notes with heavier spice. There are fewer nuances in the cup, but an intense, rich flavour.