Often referred to as 'gray dwarves,' duergar are a sub-species of dwarf in many fantasy role-playing settings. They are usually evil, and live underground (in the D&D settings this usually equates to the upper Underdark). Stereotypically they are emaciated, with gray skin and hair. Due to their sensitivity to light, they tend to stay underground and avoid daylight or bright lights as much as possible.
Duergar are referred to as the outcast dwarven sub-species, and are sometimes said to associate or cooperate with drow, the dark elves.
Some information taken from the D&D 3rd Edition Monster Manual, published by Wizards of the Coast