With adenosine blocked out, blood vessels in the brain constrict. Hormones are released, signaling the body to produce adrenaline. You become more alert, more talkative, your heart beats faster, and you feel brave enough to get to work. You get better at activities that involve endurance and large motor skills. Surgeons and sharpshooters tend to avoid caffeine before going to work.
Caffeine is a psychoactive drug, and people who use it find it habit-forming.
However, whether or not that habit qualifies as a full-fledged addiction is arguable. A few experts say yes, pointing out that a sudden reduction can cause withdrawal symptoms, such as headaches, fatigue, and drowsiness. But in the true medical sense, that's not an addiction. You'll probably be jittery and have trouble getting to sleep - although that's relative. If you need to cut down, don't try to go cold turkey.