Simply put, caffeine is an addictive drug.  It is a chemical compound, and is a central nervous system stimulant.  What this means is that it makes your heart beat faster.  In its purest form, caffeine is a bitter white powdery substance, but it doesn’t exist naturally in this form.  Pure caffeine is mainly obtained through decaffeinating the coffee bean.  Caffeine is also an alkaloid.  It enters your bloodstream in less than 15 minutes through your stomach, and can cause your blood vessels to constrict.  

Caffeine’s main source is the coffee bean, but it is also found in the tealeaf, cocoa pod, and kola nut.  Caffeine stimulates the brain the same way amphetamines, cocaine and heroin do, although at a smaller level. 

The "Discovery" of Coffee

Legend has it that coffee was "discovered" around 850 AD in upper Egypt by a goat herder named Khaldi. One night, Khaldi's goats did not return home. When he found his goats, Khaldi saw them dancing around a shrub with red berries (coffee beans). After Khaldi tried some of the berries, he started to dance too. Khaldi spoke with some monks who used the berries to make a drink and....coffee was born!
- (University of Washington)