Caffeine is a very potent stimulant. It gives a sensation of well-being [pick-me-up], it sharpens acuity and reflexes and, in sufficient quantities, can increase strength.
Caffeine, per se, is not banned by the UCI -- can you imagine the outcry if Italian and French racers were denied their espresso? But it is controlled. According to the UCI regulation, "for caffeine, a sample shall be considered as positive if the concentration in the urine exceeds 12 micrograms/ml." You'd get that by drinking eight or ten long espressos in an hour.
Everything you need to know about the UCI and doping can be found
here.