The Jem'Hadar are a hostile alien species that were introduced in the Star Trek: Deep Space Nine episode "The Jem'Hadar". They were the shock troops of the Dominion, serving the will of the Founders.
At some point in the distant past the Founders had engineered a species to serve as their warriors. It is unknown what species served as the template for the Jem'Hadar. The Jem'Hadar were designed to grow to maturity in just a couple weeks. They were dependent on a drug called Ketrecal White, which provided a critical enzyme for them without which they would eventually die. This was done to ensure the loyalty of the Jen'Hadar to the Founders. Additionally they were designed to regard the Founders as gods, and to obey their orders without question. Due to their bio-engineered nature, there were no female Jem'Hadar, all Jem'Hadar were male.
The Jem'Hadar were put under the command of the Vorta, another race who were bio engineered to serve as the diplomats and administrators of the Dominion. Groups of Jem'Hadar were led by a First, who could execute soldiers who refused to obey his orders.
Due to the violent nature of their existence, most Jem'Hadar died quite young. Few lived into their teens, and the even fewer that lived over the age of 20 were considered honored elders.
The United Federation of Planets first became aware of the Jem'Hadar in 2374, when Benjamin Sisko, Quark, Nog, and Jake Sisko encountered them on a Gamma Quadrant world. While Sisko and his companions were escaping from Jem'Hadar custody, a Jem'Hadar ship rammed the retreating Galaxy class USS Odyssey, destroying that ship.
After the Dominion allied with the Cardassians, relations between the Dominion and other Alpha Quadrant powers - specifically the Federation - deteriorated into open warfare. The Jem'Hadar served as the front line troops throughout the conflict with the Federation and its allies. Due to being cut off from Gamma Quadrant reinforcements, the Dominion began designing a specialized Alpha Quadrant version of the Jem'Hadar to fight the Alpha Quadrant races. There was considerable tension between the original Jem'Hadar and the Alpha Qudrant Jem'Hadar.
In the closing days of the war as the Cardassians turned against the Dominion, the Female Changeling ordered the Jem'Hadar to commit genocide against the Cardassians. By the time she finally surrendered, over 800 million Cardassians had died at the hands of the Dominion.
The Jem'Hadar returned to the Gamma Quadrant after the war. The few remaining renegades who disobeyed orders to return were hunted down and destroyed. Several months after the war Odo sent a Jem'Hadar named Taran'atar to DS9 to learn more about the races of the Alpha Quadrant and to act as Odo's emissary in the Alpha Quadrant. Taran'atar was one of the few Jem'Hadar who was born without a dependency on the White.