Basically energy drinks are drinks with higher quantities of sugar and caffeine (a stimulant) that supply you with an energy boost but also a lot of calories. Some of the more popular brands are Red Bull, Monster and Rockstar and should not be categorized with sports drinks such as Gatorade. Sports drinks are designed for endurance exercise and relatively low in sugar and have no stimulants. 
How Much Sugar?
Rockstar has 62 grams of sugar (nearly 16 teaspoons) and Monster Energy has 54 grams per 16 oz. can. One can of 16 oz. contains well over double the daily limits of sugar for most women and teens. These products serve no purpose whatsoever but make children addicted to caffeine and habituated to sugars according to the UK research group Action on Sugar. Even artificially sweetened versions with a cocktail of stimulants can have adverse effect on the cardiovascular system.
Undesired Effects
A Mayo Clinic study in the Journal of the American Medical Association had 25 young adults drink a 16 oz. can of Rockstar and found that after 30 minutes there blood pressure rose 6 points. Norepinephrine (a stress related neurotransmitter) also shot up. With 240 milligrams of caffeine (about 2-3 cups of coffee worth) plus some other stimulants like guarana, taurine, milk thistle and ginseng, this wasn’t unexpected.
Can It Kill You?
The high caffeine content in these drinks can certainly be harmful for some people sensitive to this stimulant as well as cardiac conditions, children and teens (especially when combined with alcohol). About two dozen deaths have been connected to high-caffeine energy drinks along with many side-effects including cardiac arrest and seizures. For more information see