Coffee can be complicated.
There are many factors that contribute to the taste in your cup, from the soil it is grown in all the way to how it is prepared before being consumed. One of these factors is the variety of coffee that is grown by the coffee farmer. It's not just about where it's grown, it's about how it's grown and what variety of coffee is chosen.
Like all living things, coffee has its own family tree. With coffee, there are two predominant 'families' who dominate the coffee lineages. You have probably heard of these: Coffee Arabica and Coffee Robusta (also called Coffee Canephora). There are other species as well, but they are not usually discussed in the coffee industry because they are not considered commercially viable.
Between Coffee Arabica and Coffee Robusta, Arabica is widely considered the prized species for coffee drinking, and is the focus of the specialty coffee industry. Arabica coffee is believed to have originated from Africa; specifically Ethiopia, Kenya and the Sudan. Today this is still one of the most highly regarded coffee growing regions in the world.
So what is a coffee varietal? If it helps to think of Arabica and Robusta as two families, then the coffee varietals are the children that have been produced, whether by design or natural evolution. The specialty coffee industry focuses on the children from the Arabica family. If you were to buy coffee from a supermarket there is a good chance it is Robusta.
Common Varieties of Coffee
Just to confuse the family tree analogy somewhat, the grandfathers of the Arabica species are the Typica and Bourbon varieties. Typica is slow growing and common in Central America, and is sweet and clean in the cup with a medium body. The Bourbon variety is common in Brazil, and is also grown in Central America. Although still a sensitive plant, it is capable of producing more fruit than the Typica variety. Bourbon coffees tend to be bright and fruity in the cup, sometimes sweet.
The Caturra variety of coffee is a natural evolution from the Bourbon variety, and not surprisingly originated in Brazil. It is commonly used throughout parts of Central America. Caturra tends not to be as sweet as Bourbon, and instead has more of a citrus acidity and medium body.
The Catuai variety has a Robusta influence, and was developed by crossing Caturra and another variety called Mundo Novo (which itself was developed from Typica and Bourbon varieties to resist disease) to produce more fruit. It originated in Brazil and is also grown through parts of Central and also South America.
Ethiopian Heirlooms is the term used to describe the wild varieties of coffee plants that grow in Ethiopia. Scientists are still piecing together the family tree, but the Sidamo and Yirgacheffe varieties have become well known and highly regarded. Flavour can vary enormously, from a light floral tea to a deep spicy chocolate.
Gesha (often written incorrectly as Geisha) is highly regarded in the specialty coffee industry, made even more so due to its reluctance to grow. In the cup it is sweet and citrusy. It is now championed by Panama and has to be grown at high altitude. No one is really sure how it originated, although it's likely a natural evolution of the Typica variety out of Ethiopia.
Coffee farmers would commonly select coffee plant varieties based predominantly on what they could grow the most of with less risk of loss due to disease or weather conditions. With the influence of the specialty coffee industry this has started to change, with an increasing number of coffee farmers willing to focus exclusively on the quality of the cup.