The Jem'Hadar are a genetically-engineered alien race that appear throughout much of Star Trek: Deep Space Nine. They are the soldiers of the Dominion, an expansionist military power from the Gamma Quadrant that has dedicated itself to bringing order to the universe through conquest. Jem'Hadar appear as grey-skinned, reptilian humanoids that possess great physical strength and are genetically programmed to function as effective, ruthless fighting machines. Their obedience to the Founders is ensured thanks to a narcotic compound - ketracel-white - which their bodies have been designed to be completely dependent on. White withdrawal will make a Jem'Hadar go insane, causing it to turn rabid and viciously attack anything and anyone in its path.
Jem'Hadar are excellent soldiers; they possess strength many times that of humans, wield finely-crafted bladed weapons and their plasma-disruptor weaponry has a malicious side effect that prevents a victim's blood from coagulating, ensuring that every shot is fatal since a victim will inevitably bleed to death. Jem'Hadar are also resistant to energy fields and have been known to bypass force fields. One of their most impressive traits is the ability to "shroud" themselves, camouflaging themselves almost perfectly by blending into the surrounding environment.
The Jem'Hadar first appear in the DS9 Season 2 episode "The Jem'Hadar". After the DS9 crew's first encounter with them, the Federation quickly comes to recognise the threat that the Dominion represents to the Alpha Quadrant and there are numerous bloody encounters with the Jem'Hadar throughout the rest of the series, most notably in the sixth and seventh seasons when the Federation and the Dominion engage in full-scale war with one another.