Coffee beans are actually seeds from a cherry of the coffee tree. There are two major types of coffee beans Arabica and Robusta.Robusta plants yield much more than Arabica, contains more caffeine and is less susceptible to disease as well. It has a reputation for being an exceptionally strong coffee with a more bitter flavour than Arabica. Due to its pros (yield, easier to look after, strength) and its cons (bitter flavour) Robusta is more commonly used in low-end coffee and as a blend. However, good quality Robusta beans are starting to be explored for use in great coffee.
Arabica beans, as mentioned above, are considered to be of a greater quality bean due to the perceived richer taste and lower caffeine (and lower bitterness). The beans themselves are typically oblong and roughly 1cm in length.