The Jem'Hadar are an alien race on Star Trek: Deep Space Nine. They were engineered by the Founders (of the Gamma Quadrant's Dominion) with a variety of special features:
Of primary importance is their requirement of an isogenic enzyme called Ketracel White to survive. The White is synthesized by the Founders and given to the Vorta, which allows the Vorta to control their troops.
In addition, the Jem'Hadar are also fanatically loyal to the Founders - they see the founders as gods. This helps keep the Vorta in line, as the Jem'Hadar would rather die than allow the Vorta to act counter to the will of their gods.
Another nifty feature is the ability to cloak themselves at will. Sadly for them, they do not reproduce in the normal manner, and are grown in birthing chambers. Presumably they would therefore have no desire to use their abilities for their own amusement, however. Instead of thinking dirty thoughts, they have an inborn propensity for violence. Well, maybe that just follows from having no sexual organs.
Because they are grown from a standard genetic stock, there is also no chance of accidental genetic drift. The only significant exception is that guy in the Season 4 episode Hippocratic Oath who was neither addicted to White nor irrationally violent.
After the Jem'Hadar fleet was destroyed by the prophets, the Founders produced a new breed of Jem'Hadar, ostensibly better suited to combat in the Alpha Quadrant. Just what is different about them, apart from pigmentation and pointiness, I don't know. There was still a great deal of tension between the Alphas and the Gammas (Those from the Gamma Quadrant). A good example is One Little Ship (DS9) (even though it hasn't been noded yet).
The leader of a Jem'Hadar unit is called a First, while all others have similar designations - Second, Third, and so on. When a Jem'Hadar dies, all others below him move up the ladder.
The Jem'Hadar are quite important in the DS9 world, both as the muscle of the Dominion, and to show us just how the Founders feel about solids. The Founders make a show of appreciating the fealty of their minions, but also call them in to existence for their own uses and allow them to die on a whim. | 
