A common dose of Caffeine is around 50-150mg. Roughly this much Caffeine can be contained in an average sized coffee (7 oz.) or three cans of Cola. Majority of people take this much caffeine daily. A common dose grants most of the positive effects of caffeine (increased focus, memory and a more awake feeling) with little negative effects. Another reason why this is called a common dose is because it prevents your body from going into withdrawal, if you are a chronic caffeine consumer. For daily use this is the most recommended dose.
If you're heavily sleep deprived, or need to perform at your absolute best for an important event, it might be beneficial to take a "Moderate" dose of caffeine (150-400mg). The effects from this amount of caffeine become much more noticeable. At this level you should be feeling wide awake, and almost jittery with all the energy. With all this caffeine your body will be producing significantly more dopamine and adrenaline, which are important hormones when it comes to being at your best. The drawback is the recovery time your body will need after the drug wares off. It is important to take a break to let your nervous system reset after such a stressful event. In summary a moderate dose is ok occasionally, and after such a dose it is wise to back off caffeine to let the body recharge.
Problems start to show up with continual use moderate or higher level doses. This level of dose keeps your body in a state of elevated stress levels, and can lead increased tolerance as well as addiction. At this level your body will start to require more caffeine in order to reap the same benefits, and become dependent on the substance in order to function.
This is when people commonly start taking Heavy doses of caffeine. A Heavy dose is over 400mg of caffeine. This equals around 3 cups of typical coffee. This much caffeine is excessive when you are trying to get the performance benefits of caffeine, and the only reason this much caffeine should be needed is if you're tolerance has been raised higher than average due to repeat consumption. If you find yourself needing to take over 400mg caffeine to feel the effects, or even to merely avoid the symptoms of withdrawal, you should start limiting your consumption.
Finally I'm sure you're curious as to how much caffeine it would take to kill a person. The Lethal dose ranges from 3-20 grams. A dose of this volume has a potential to kill or warrant a trip to the hospital. It would be very difficult to have this much caffeine in a day, and deaths by overdose of caffeine are very rare.