Beans designated for espresso are roasted longer than regular coffee beans, so that the oils are brought to the surface of the bean. The selection of the best type of bean is open to debate, although Arabica beans are credited with providing the most balanced compromise between flavor, acidity and bitterness. Robusta beans, although attributed to being a lower quality bean, provide more crema (the reddish-golden creamy foam floating on the top of a perfect espresso), and make an excellent augmentation to Arabica beans.
Without crema, espresso is nothing but thick, strong coffee. The crema also slows the dispersion of the volatile oils into the air after brewing, contributing to the distinctive aroma of espresso.
For the best espresso, try to use the freshest beans. Old beans may appear very oily, and have a stale odor. You should always store your beans in an airtight container in a dark, dry place. Do not store beans in the freezer, as this will crystallize the moisture, and deteriorate the porous structure of the beans, which may result in a lower quality flavor.