Changelings are an Aliens from the Gamma Quadrant who control vast swathes of that Quadrant. They appear primarily in Star Trek - Deep Space Nine as the main antagonists. Changelings are an aggressive race and ones that are at the seat of power of the Dominion. Changelings are liquid aliens as opposed to solid aliens like Klingons and Vulcans. The Changelings are able to transform themselves into any shape that they desire.
The Changelings home world is a planet that orbits no Star, such planets are known as Rogue Planets. The planet is located in the fictional Omarian Nebula in the Gamma Quadrant. The Changelings on the planet live in a Great Sea containing many millions of them. The planet is attacked by the Romulans but Jem'Hadar warships are able to repulse the attack but not before the Romulans devastate the planet.
Changelings are able to take away the ability to shape shift from one another which they do to Constable Odo after he kills a fellow Changeling.
The Changelings have created their own army race, the Jem Hadar as their foot soldiers. Apart from the Changelings, the Vorta are also part of the Dominion but subservient to the Changelings. The Cardassians joined with the Dominion for the length of the Dominion Wars until a peace treaty was signed. The Changelings are also known as the Founders primarily by the Jem Hadar.