Arabica is a variety of coffee plant from which coffee beans are harvested; in the same way that Shiraz is a variety of vine from which grapes are harvested for producing wine. There are several varieties of coffee plant, but only two varieties are grown commercially: Arabica and Robusta.
Arabica tends to be a more successful crop when grown at altitude, while Robusta typically us more sucesful in wider variety of environments. Robusta also tends to be a more "robust" crop.
It is generally accepted that coffee beans from the Arabica variety produce roasted coffee with more desirable flavour characteristics with a lower caffeine content compared to coffee beans from the Robusta plant variety. However, however Robusta is valued for its higher oil content, this oil content provides superior crema production (crema being an emulsion or oil and water created at temperature and under pressure).
It is very popular with leading blends of roasted coffee beans to incorporate Robusta coffee beans blended into their Arabica coffee bean blend. The addition of Robusta into the blend seeks to both balance the acidity of the Arabica blend while increasing the oil content and hence the crema of the blend when used to make espresso coffee.
100% Arabica is used to refer to coffee where the coffee bean blend has been formulated by the roasteing company without the addition of Robusta coffee beans.