Caffeine is the key active chemical, an alkaloid, in coffee, cola drinks and tea (where it may be called theine). Caffeine acts to stimulate the nervous system, raise the blood pressure and avert tiredness or fatigue. Excessive amounts can cause insomnia and nervousness, plus it acts as a diuretic and so increases the flow of urine.
The level of caffeine needed to have an effect is between 100 - 300 mg. The quantities of caffeine in different drinks include: 100 - 150 mg in coffee; 50 mg in cocoa, cola drinks 35 - 55 mg per can and tea 50 - 100 mg.
Coffee and tea can be treated to remove caffeine to render them "decaffeinated". Decaffeination is carried out by either the Swiss water process or the carbon dioxide (CO2) processes. The Swiss water process relies on the fact that hot water removes caffeine quickly, so you can reduce the caffeine significantly yourself, by infusing for 30 seconds, discarding the brew and then reinfusing it with more boiling water.