Arabica and robusta are the two main types of beans for all coffee. Robusta beans are less expensive and easier to grow. Arabica beans tend to make better coffee. Roasting is what turns green beans into coffee that is ready to grind and brew.
Light roasting produces beans that are light brown and have a more sour taste.
Medium roast coffees have medium brown beans. The beans do not have an oily surface in this roast. The coffee beans can have a bright acidity, but specific varietal aromatics (e.g. floral, fruity, vegetable, berry, etc.) of the coffee are still apparent.
The beans in this roast have some oil on the surface and the color is rich and darker. The characteristics of the coffee are complemented by caramelization notes such as nutty, bread or baked goods, or chocolate, and the acidity has faded somewhat, bringing out a slightly bittersweet aftertaste. French roast is a good example.
The darkest roasts have shiny black beans with an oily surface. In a good/well done dark roast, there is still some good acidity to liven the cup. Dark roasts run the gamut from slightly dark to extremely charred. Italian roast and French roast are darker roasts.