At its most basic caffeine is a molecule made up of hydrogen, carbon, nitrogen and oxygen. It is the result of evolution, an attempt by the coffee plant to protect itself from insects.
Caffeine enters the bloodstream and quickly finds its way to the brain, where it works as a stimulant, increasing alertness and energy. It can also impact your mood by raising the levels of feel-good chemical dopamine in your brain.
It can take as few as 10 minutes for caffeine to start working and can remain in the body for up to 6 hours (this is known as its half-life).
At 400 billion cups a day coffee is the worlds most consumed beverage and without the stimulating effects of caffeine there is little doubt it would not be so widely drank as it is today.