Changelings are a shape-shifting species, over the years several types of changelings have been encountered.
The first shapeshifting species to be encountered was a race of solid life forms that could change their form at will, one member of this species was hired to kill Captain James T. Kirk and Doctor Leonard H. McCoy after they were framed for the murder of Chancellor Gorkon and sent to the Klingon prison world of Rura Penthe (TOS Movie Star Trek VI: The Undiscovered Country)
Another species of changeling is one that lives in the Gamma Quadrant. In their most basic state Changelings resemble a golden translucent liquid and they must revert to this state every sixteen hours to regenerate. Changelings can take on almost any form; liquid, solid, gas and even plasma such as flames. Their abilities are limited mainly by their mass: they comfortably take on humanoid forms, but can also take the forms of things somewhat larger, and a great deal smaller. They are the leaders of the Dominion and are referred to as the Founders (DS9 episode The Jem'Hadar)