A gravitational field affects anything that has mass or energy. This includes visible matter, dark matter and light. The strength of a gravitational field between two objects is directly proportional to the amount of mass they have and inversely proportional to the distance between them. For energy, the treatment is the same.
Isaac Newton characterized a gravitational field as a force between two point masses. He observed an apple falling from a tree and realized that this described the motion of the moon with respect to the Earth.
Albert Einstein described the gravitational field in terms of geometry. A concentration of mass or energy curves the fabric of space-time. The gravitating substance – and nearby collections of mass and energy – follow this curvature in what looks like an orbit.
Gravitational fields are responsible for the formation of planets and the life and death of stars and galaxies. Gravity is also responsible for keeping us bound to the Earth, because we don't have the energy to escape it. Rockets break free by expending enough fuel to reach a speed sufficient to break free of the Earth's pull. Black holes are objects with gravitational fields so powerful that not even light can escape.