Caffeine actually binds to those receptors in efficient fashion, but doesn’t activate them—they’re plugged up by caffeine’s unique shape and chemical makeup. With those receptors blocked, the brain’s own stimulants, dopamine and glutamate, can do their work more freely […]
In the book, Braun ultimately likens caffeine’s powers to “putting a block of wood under one of the brain’s primary brake pedals.”

— Kevin Purdy (2010) “What Caffeine Actually Does to Your Brain” (and updates)
This is an interesting post about the effects of caffeine on the body, citing the book “Buzz” by Stephen R. Braun, with some correction and updates in the comments.
A few useful details about caffeine:
  1. Caffeine doesn’t accelerate anything, instead it avoids the brain to go sleepy.
  2. It helps keeping the speed, not the accuracy: “think speed, not power”.
  3. The body becomes tolerant to the daily dose of caffeine, regardless of quantity, in 7 to 12 days.
  4. Caffeine withdrawal happens after 12 to 24 hours (it coincides with the next morning coffee).
  5. Caffeine withdrawal symptoms could be: headaches, depression, fatigue, lethargy, irritability, nausea.
  6. The body restores from the withdrawal in around 10 days.
(hat tip to Mark for the link)