Also known as 'Changelings' or 'Shapeshifters',  the Founders are a shape shifting species based in the gamma quadrant.  Physically, in their natural state they resemble a very viscous liquid. A Founder can alter its shape,  size and mass,  colour and texture at will. Indeed, an experienced Changeling can imitate almost any object - even mist and fire!  Young and inexperienced Changelings, however, find creating a perfect copy of an existing person or object difficult and rarely produce an accurate copy. 
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In addition to their shape shifting abilities, Founders are capable of merging with each other when in their liquid state. They call this process 'the Great Link', and it allows the Founders to achieve a considerable degree of information and memory sharing while increasing their ability to modify their own internal structure. On the Founders home world many millions of individuals spend virtually their entire time within the great link, which is comparable to an ocean in size. 
In their history the Founders have often been persecuted by non-shapeshifters, whom they refer to as 'the solids'. In order to guarantee their safety they created the Dominion - a grouping of many solid species under the control of the Founders.  The Founders have resorted to genetic engineering in order to modify several of these species to increase their utility to the Dominion. - most notably the Vorta. Thousands of years ago a Changeling was given refuge from an angry mob by the forest-dwelling Vorta. So grateful was the Changeling that it arranged for the Vorta to be genetically enhanced, turning them into the Founder's main representatives within the Dominion.  The Jem'Hadar were also extensively modified to make them into the perfect soldiers for the Dominion. 
Although individual planets within the Dominion are treated reasonably well, disobedience to the Founders is dealt with extremely harshly - it has been known for entire planets to be infected with deadly diseases for rebelling against Dominion rule. 
The Founders attempt to expand their influence into the Alpha Quadrant was the root cause of the Dominion war.  Although the Dominion initially made large inroads into the Quadrant,  the Federation eventually led an alliance which was able to defeat them.  During the war the organization known as Section 31 was able to infect the Changeling Odo with a deadly disease,  which was transmitted through the great link to all other Founders.  Fortunately, Dr. Bashir was able to cure the disease  and on the eve of the final battle of the Dominion war Odo agreed to return to the great link with the cure in return for the Dominion's surrender.