A graviton field is any area in which many gravitons may be found. Graviton fields are used in the production of artificial gravity. They are also found naturally-occurring in space.
Generally speaking, natural graviton fields appeared to be more powerful than man-made fields such as the one encountered by the USS Enterprise-D in 2367. (TNG: "The Loss")
Later that year, the Cytherian-influenced Reginald Barclay modified the Enterprise nacelles to create a highly charged graviton field, which created a subspace distortion and enabled the ship to travel to the Cytherian homeworld. (TNG: "The Nth Degree")
In 2369, the graviton field of Deep Space 9's tractor beam was able to stabilize Tosk's ship's structural integrity. (DS9: "Captive Pursuit")
Several weeks later, a lifeform from the Gamma Quadrant created a graviton field that drew DS9 towards the wormhole. (DS9: "Q-Less")
The graviton field output of the shields of the Galaxy-class USS Enterprise-D was 625 MCH. (Star Trek Generations)
The USS Voyager encountered a natural graviton field in 2373. (VOY: "Future's End")
Lenara Kahn and the Trill Science Ministry came up with a way to seal the Bajoran wormhole using a modulated graviton field. (DS9: "In Purgatory's Shadow")