The flavour and caffeine strength of coffees and are strongly influenced by the skill and duration involved in the roasting process. This relates partly to the behaviour of the volatile aromatic compounds in coffee which give it aroma and flavour. The roasting process significantly increases the volume of these compounds, thus enhancing the flavour. However, as roasting periods length the flavour of the bean is over-taken by the taste of the roasting itself.
Here is a brief outline of the different types of roasts available:
Light roast: Used for milder flavour, this type of coffee can taste almost like wheat or grain. The surface of the bean is light brown, with no visible oil.
Medium roast: With a stronger flavour a bit sweeter than light roast. A full body is balanced by an acid snap, with aroma and flavour complexity. The beans look medium brown, but still have no visible oil sheen on them.
Dark roast: Richer and darker, flavour complexity is traded for rich chocolaty body and aroma is exchanged for sweetness. These beans will have a slightly shiny look from the coffee oils.
Darkest roast: Shiny black beans, they are smokey with some bitterness and taste primarily of roasting rather than the inherent flavour of the beans. These beans are used mainly for espresso.
It's a common misconception that caffeine strength correlates to the darkness of the roast. But in fact, lighter roast coffees tend to preserve the caffeine in the beans. If you seek a strong caffeine kick, look for a coffee which contains Robusta beans and which is a light or medium roast.