The Dominion is a group of three main races: The Founders (Shapeshifters), the Vorta, and the Jem Hadar. The Founders are in control of all of these races. All below them must obey and folow their orders. The Vorta are a race created by the founders. They command the ships, and give out the rations of Ketracel White, which the Jem Hadar absolutely need to survive. The Jem Hadar are the workers for all of them. They are warriors that fight only for the founders. Their common saying  is "Victory is Life". The Dominion have been involved in only one major war. They started it.  When allied with the Cardassions, they were sending ships through the Bajoran Wormhole. The Deep Space Nine Space Station commanded by Benjiman Sisko, didn't like the idea of thousands of ships pouring through the wormhole, so he mined it. This pretty much started the war. In the end, the Breen joined their alliance, and they ended up losing the war because the Cardassions joined the Federation at the last second.