Critical hit does double damage the move normally would have done, unless you have a pokemon with the ability sniper, then it does 3 times the damage.
X2 Damage. For example if you used ........FLy with a base power of 90 it would be doubled to 180 power.
The damage inflickted is doubled. If the move is also Super Effective, it doesnt rule out critical hits. 3X damage!!
Even BETTER if it is STAB, 3.5 power!
Abilities and Items can also increase the damage such as Hustle, or Menronome.