Caffeine is a naturally-occurring stimulant which
absorbed in around 45 minutes.
It's typically found in, Food (Chocolate), Drink (Tea & Coffee/Energy Drinks), Tablets, Powder

You body builds a tolerance to it over time therefore you will need to consume more for the same effect.
A great tactic to maintain the relative benefits is to have one or two days off all caffeine per week.  

Caffeine can effect sleep due to its half life of 4-6 hours.  If you consume 100 mg in 4+ hours there is still 50 mg of caffeine in your body.  A good practice is to stop consuming around midday.
Other things to note:

You can become dependent on it.
Is a mild diuretic.
Small increases in metabolism.
Can reduce appetite in some.

Sport performance benefits -

Spares muscle glycogen
Increases muscle fibres excitability
Reduces reaction time
Increases alertness & improves decision making, especially when sleep-deprived
Improves muscular endurance
Improves endurance performance