Genetically engineered from original unknown stock: the Jem'Hadar live to serve the Founders of the Dominion. Their sole concern is combat. Conceived in and born from incubation pods, so there is no need for female Jem'hadar. These infants mature into a battle-ready adult it as few as three days.
To keep them in line, the Founders bred them to be addicted to the drug Ketracel-White. A vial of Ketracel-White hangs from a Jem'Hadar's colar, and a tube from the vial pumps it straight into his jugular.
The Ketracel-White provides them with all of the nutrition they need, so they do not eat. They believe that rest is a sign of weakness and would make them soft, so they never sleep. They do not 'recreate' or 'relax' (apart from simulated combat training, which they treat with deadly seriousness and don't really enjoy), so they do not relax.
They do not sleep. They do not east. They do not recreate. There are no female Jem'Hadar. The sole occupation of their time is combat.
Jem'Hadar spend a life in combat, and there is a high mortality rate, but they can always grow more: few Jem'hadar live to be 15 years old, none have lived to be 30. Those Jem'hadar that live 20 years reach the rank of "Honored Elder".
The Jem'Hadar are also bred to revere the Founders of the Dominion (a race of shapshifters) as gods, although the Founders are rarely seen.
The Dominion has a three-tiered command structure: At the top are the Founders and at the bottom are the Jem'Hadar. Serving as intermediaries between them are the Vorta, another race genetically engineered by the Founders. Vorta supervisors dispense out new Ketracel-White vials to the Jem'Hadar, and serve as diplomats, supervisors, and go-betweens within the Dominion.
Jem'Hadar ranks are fairly simple: the highest in rank is refered to as "First", the second in commmand, "Second", and the Third, "Third" (as in, "Third Remat'a'klan) and so on.
The Jem'Hadar are bred to believe that thier sole purpose in life is to fight for the Founders. Unlike Klingons, they aren't really looking for an honorable death: they must serve the Founders; if successfully completing a mission for the Founders means sacrificing themself, they will do it without hesitation, but they would generally count their own death as a failure to the Founders if they did not succeed. "I serve the Founders in all things" is the idea.
Before a battle, the ranking Jem'Hadar will solemnly recite to those under his command the Jem'Hadar Battle Dirge:
Ranking Jem'Hadar:"I am (ranking Jem'Hadar's name), and I am dead. As of this moment, we are all dead. It is in Victory for the Founders that we attain life. Victory is life. We do this GLADY, because we are Jem'Hadar. Remember; Victory is life!"
Gathered Jem'Hadar: "Victory is life! Victory is life! Victory is life!"